[h=3]By JOE PARKINSON in Reyhanli, Turkey, and SAM DAGHER in Beirut[/h]Syria's government sent significant reinforcements toward its most-populous city on Wednesday in an effort to dislodge rebels battling government troops there, activists said, in what some observers said could shape up to be a pivotal battle in the 18-month uprising against President Bashar al-Assad.
Fighting continued for a fifth day in the city, Aleppo, a commercial hub less than 40 miles from Turkey's border. Turkey, citing worsening security conditions along the frontier, announced Wednesday that it would seal its border crossings with Syria, although the frontier would remain open to refugees fleeing violence.

AFP/Getty ImagesRebels hunted for snipers in Selehattin, near Aleppo, Monday.
Syrian rebel fighters retained control Wednesday of three border crossings they seized from Syrian government forces last week, Turkish officials and opposition activists confirmed.
Syrian government troops appeared to be concentrating further south in Aleppo. The regime dispatched nearly 80 tanks Wednesday from a province east of the city that has largely remained loyal to Mr. Assad, two opposition activists said, citing reports from their networks.
Until recent days, the largely mercantile Aleppo had been spared the scale of violence that has beset the opposition hubs of Homs and Hama. But the revolt against Mr. Assad's regime has morphed from an insurgency concentrated in the provinces into a battle for control of Aleppo as well as the capital, Damascus, where fighting intensified dramatically last week.
In Damascus, government officials acknowledged the fighting in the north. But they said the relative calm that has returned to the capital is an indication that the military can keep control of major cities. "The army is doing its job there, like it did here," one ministry official said.
Rebels in the three Aleppo neighborhoods—Salahuddin to the southwest and al-Sakhour and al-Sha'ar to the northeast—were fortifying their positions and ranks, residents said, preparing to face off with government troops these people said have encircled all three areas.
Rebels have positioned heavy machine guns and antiaircraft guns on rooftops in Salahuddin, one resident said.
The White House condemned the "heinous violence" of government attacks on Aleppo, saying it had credible reports that pro-regime forces were attacking not only with helicopters and tanks, as it has in past attacks in civilian areas, but also with fixed-wing aircraft.
"It's just another indication of the depths of depravity that Assad has demonstrated himself capable of achieving," White House spokesman Jay Carney told reporters aboard Air Force One.
A dramatically increased flow of Aleppo residents are fleeing violence in the city, Syrian activists in Turkey said. Injured rebel fighters were also taking refuge in the Turkish border town of Reyhanli, which houses one of the 10 refugee camps on Turkish territory that host close to 45,000 Syrian refugees in all.
"We have high morale, but the government is using fighter jets and heavy weapons," said Abu Yazzam, a 28-year-old fighter from Aleppo who arrived in Reyhanli late Tuesday. He said he broke his arm fleeing government sniper fire in Salahuddin, where he had been based with other opposition fighters who had taken control of the district.
"I will go back to fight when my arm heals," he said in an interview in a safe house.
High morale notwithstanding, the remaining rebels are likely to be outnumbered and outgunned, some analysts said.
"For a very long time the argument was that as long as the demonstrations had not reached Aleppo and Damascus, the regime will prevail. But now we appear to have seen a turning point," said Sinan Ulgen, a middle east analyst at the Carnegie Endowment for International Peace. Despite the discrepancy between government and rebel capabilities, he said, "the battle is unlikely to be over quickly."
Other accounts suggested that there were divisions within rebel ranks. Most of the rebels holding the three Aleppo neighborhoods are fighters from the surrounding countryside and neighboring Idlib province who have battled regime forces there for months, said Fadi Salem, a resident of the United Arab Emirates who said he was visiting family in Aleppo.
Mr. Salem, who said by telephone from U.A.E. that he visited Salahuddin before leaving Aleppo on Wednesday, said turf battles were already breaking out among rebels there and that residents had mixed feelings about being caught in the middle of an all-out-battle. The densely populated, working-class neighborhoods were already hosting families that had escaped the fighting in Idlib and the Aleppo countryside since the start of the conflict 18 months ago.
Citing the deteriorating security situation south of the border, Turkish economy minister Zafer Caglayan said Wednesday that Ankara would halt the passage of all commercial vehicles from Turkey into Syria.
Announcing Wednesday's border closure to commercial traffic, Turkey's economy minister cited "serious concerns" for drivers' safety and noted that there had already been a 87% drop this year in trucks traveling to Syria. Crossing the border has become increasingly dangerous for truck drivers as Syria's conflict has spread. Turkish media have carried numerous reports of drivers being targeted by combatants or caught in the crossfire.
Before the Syria conflict, the main border gate here at Reyhanli, across from Bab al-Hawa, Syria, was typically clogged with trucks transporting food, consumer goods and textiles. It was quiet Wednesday save for a trickle of Syrian trucks, which Turkish officials allowed to pass through.
For years, Ankara invested heavily in strengthening ties with Mr. Assad's administration in a bid to boost trade and help foster change in Syria. The closure of the border gates marks the latest hit for a commercial route that helped propel robust economic growth in this border town in the decade before the uprising.
In Antakya city, hundreds of new and used articulated trucks sit gathering dust at garages.
Ali Harbelioglu said he was forced to quit his job as a truck driver last week after 10 years transporting Turkish goods to Saudi Arabia through Syria.
"I was in Syria 10 days ago, but now there's no work. I'll be driving for a travel company," he said. "There are hundreds like me who are now jobless."
—Nour Malas in Damascus and Ayla Albayrak in Istanbul contributed to this article.Write to Joe Parkinson at [email protected] and Sam Dagher at [email protected]
Fighting continued for a fifth day in the city, Aleppo, a commercial hub less than 40 miles from Turkey's border. Turkey, citing worsening security conditions along the frontier, announced Wednesday that it would seal its border crossings with Syria, although the frontier would remain open to refugees fleeing violence.

AFP/Getty ImagesRebels hunted for snipers in Selehattin, near Aleppo, Monday.
Syrian rebel fighters retained control Wednesday of three border crossings they seized from Syrian government forces last week, Turkish officials and opposition activists confirmed.
Syrian government troops appeared to be concentrating further south in Aleppo. The regime dispatched nearly 80 tanks Wednesday from a province east of the city that has largely remained loyal to Mr. Assad, two opposition activists said, citing reports from their networks.
Until recent days, the largely mercantile Aleppo had been spared the scale of violence that has beset the opposition hubs of Homs and Hama. But the revolt against Mr. Assad's regime has morphed from an insurgency concentrated in the provinces into a battle for control of Aleppo as well as the capital, Damascus, where fighting intensified dramatically last week.
In Damascus, government officials acknowledged the fighting in the north. But they said the relative calm that has returned to the capital is an indication that the military can keep control of major cities. "The army is doing its job there, like it did here," one ministry official said.
Rebels in the three Aleppo neighborhoods—Salahuddin to the southwest and al-Sakhour and al-Sha'ar to the northeast—were fortifying their positions and ranks, residents said, preparing to face off with government troops these people said have encircled all three areas.
Rebels have positioned heavy machine guns and antiaircraft guns on rooftops in Salahuddin, one resident said.
The White House condemned the "heinous violence" of government attacks on Aleppo, saying it had credible reports that pro-regime forces were attacking not only with helicopters and tanks, as it has in past attacks in civilian areas, but also with fixed-wing aircraft.
"It's just another indication of the depths of depravity that Assad has demonstrated himself capable of achieving," White House spokesman Jay Carney told reporters aboard Air Force One.
A dramatically increased flow of Aleppo residents are fleeing violence in the city, Syrian activists in Turkey said. Injured rebel fighters were also taking refuge in the Turkish border town of Reyhanli, which houses one of the 10 refugee camps on Turkish territory that host close to 45,000 Syrian refugees in all.
"We have high morale, but the government is using fighter jets and heavy weapons," said Abu Yazzam, a 28-year-old fighter from Aleppo who arrived in Reyhanli late Tuesday. He said he broke his arm fleeing government sniper fire in Salahuddin, where he had been based with other opposition fighters who had taken control of the district.
"I will go back to fight when my arm heals," he said in an interview in a safe house.
High morale notwithstanding, the remaining rebels are likely to be outnumbered and outgunned, some analysts said.
"For a very long time the argument was that as long as the demonstrations had not reached Aleppo and Damascus, the regime will prevail. But now we appear to have seen a turning point," said Sinan Ulgen, a middle east analyst at the Carnegie Endowment for International Peace. Despite the discrepancy between government and rebel capabilities, he said, "the battle is unlikely to be over quickly."
Other accounts suggested that there were divisions within rebel ranks. Most of the rebels holding the three Aleppo neighborhoods are fighters from the surrounding countryside and neighboring Idlib province who have battled regime forces there for months, said Fadi Salem, a resident of the United Arab Emirates who said he was visiting family in Aleppo.
Mr. Salem, who said by telephone from U.A.E. that he visited Salahuddin before leaving Aleppo on Wednesday, said turf battles were already breaking out among rebels there and that residents had mixed feelings about being caught in the middle of an all-out-battle. The densely populated, working-class neighborhoods were already hosting families that had escaped the fighting in Idlib and the Aleppo countryside since the start of the conflict 18 months ago.
Citing the deteriorating security situation south of the border, Turkish economy minister Zafer Caglayan said Wednesday that Ankara would halt the passage of all commercial vehicles from Turkey into Syria.
Announcing Wednesday's border closure to commercial traffic, Turkey's economy minister cited "serious concerns" for drivers' safety and noted that there had already been a 87% drop this year in trucks traveling to Syria. Crossing the border has become increasingly dangerous for truck drivers as Syria's conflict has spread. Turkish media have carried numerous reports of drivers being targeted by combatants or caught in the crossfire.
Before the Syria conflict, the main border gate here at Reyhanli, across from Bab al-Hawa, Syria, was typically clogged with trucks transporting food, consumer goods and textiles. It was quiet Wednesday save for a trickle of Syrian trucks, which Turkish officials allowed to pass through.
For years, Ankara invested heavily in strengthening ties with Mr. Assad's administration in a bid to boost trade and help foster change in Syria. The closure of the border gates marks the latest hit for a commercial route that helped propel robust economic growth in this border town in the decade before the uprising.
In Antakya city, hundreds of new and used articulated trucks sit gathering dust at garages.
Ali Harbelioglu said he was forced to quit his job as a truck driver last week after 10 years transporting Turkish goods to Saudi Arabia through Syria.
"I was in Syria 10 days ago, but now there's no work. I'll be driving for a travel company," he said. "There are hundreds like me who are now jobless."
—Nour Malas in Damascus and Ayla Albayrak in Istanbul contributed to this article.Write to Joe Parkinson at [email protected] and Sam Dagher at [email protected]