24 August 2013 Last updated at 18:39 ET
Medical charity Medecins Sans Frontieres says patients' symptoms in eastern Damascus point to the use of "neurotoxic" agents
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David Cameron and Barack Obama have threatened a "serious response" if it emerges that Syria has used chemical weapons, Downing Street has said.
Activists say attacks were launched against rebels in Damascus last week.
The UK PM and the US president spoke on the phone for 40 minutes earlier.
Both were "gravely concerned" by "increasing signs that this was a significant chemical weapons attack carried out by the Syrian regime against its own people", No 10 said.
"The UN Security Council has called for immediate access for UN investigators on the ground in Damascus," the Downing Street statement said.
"The fact that President Assad has failed to co-operate with the UN suggests that the regime has something to hide.
"They reiterated that significant use of chemical weapons would merit a serious response from the international community and both have tasked officials to examine all the options."
Meanwhile, medical charity Medecins Sans Frontieres says hospitals it supports in Syria have treated about 3,600 patients with "neurotoxic symptoms", of whom 355 have died.
The medical charity, which says it cannot "scientifically confirm" the use of chemical weapons, said the patients had arrived in three hospitals in the Damascus area on Wednesday.
Both sides in the conflict accuse each other of using them.
Continue reading the main story
David Cameron and Barack Obama have threatened a "serious response" if it emerges that Syria has used chemical weapons, Downing Street has said.
Activists say attacks were launched against rebels in Damascus last week.
The UK PM and the US president spoke on the phone for 40 minutes earlier.
Both were "gravely concerned" by "increasing signs that this was a significant chemical weapons attack carried out by the Syrian regime against its own people", No 10 said.
"The UN Security Council has called for immediate access for UN investigators on the ground in Damascus," the Downing Street statement said.
"The fact that President Assad has failed to co-operate with the UN suggests that the regime has something to hide.
"They reiterated that significant use of chemical weapons would merit a serious response from the international community and both have tasked officials to examine all the options."
Meanwhile, medical charity Medecins Sans Frontieres says hospitals it supports in Syria have treated about 3,600 patients with "neurotoxic symptoms", of whom 355 have died.
The medical charity, which says it cannot "scientifically confirm" the use of chemical weapons, said the patients had arrived in three hospitals in the Damascus area on Wednesday.
Both sides in the conflict accuse each other of using them.