The medical aid organisation Medecins Sans Frontieres says it treated 3,600 people for 'neurotoxic symptoms' in three hospitals in Damascus following an alleged chemical weapons attack on Wednesday, of whom 355 died.
It is believed to be the first independent confirmation that chemical weapons were used in the attack.
The announcement comes as tensions mount in the region, with the Syrian government, supported by Russia, denying responsibility and blaming rebels, while the rebels, supported by Britain, claim Assad's forces are responsible for the atrocity.
The UN's disarmament chief, Angela Kane, has arrived in Damascus today to push for access to the site of the attacks, with Moscow supporting calls from Europe, the Middle East and the US for the Syrian government to allow UN inspectors freedom to investigate.
So far, Assad has not responded to requests for UN access to the site.
A report on Syrian state TV claims that soldiers investigating a rebel held tunnel in the Jobar area of Damascus today came across chemical agents.
"Army heroes are entering the tunnels of the terrorists and saw chemical agents," state television quoted a source as saying. "In some cases, soldiers are suffocating while entering Jobar."
"Ambulances came to rescue the people who were suffocating in Jobar," says the report, which claims the soldiers were preparing to attack a rebel-held position.
Iran has also declared that it believes rebel terrorists were responsible for the attack.
Abbas Araqchi, Iran's foreign ministry spokesman, said: "There is evidence that this action was carried out by terrorist groups.
"The concurrence of the use of these weapons with the presence of UN inspectors is itself an indication that there are hands at work to accuse the Syrian government of using these weapons and help the conflict and crisis to continue."
On Friday, US defence secretary Chuck Hagel said that he had been asked to draw up a list of military options by President Obama as the US considers it options.
"The defence department has responsibility to provide the president with options for all contingencies," he told reporters.
"That requires positioning our forces, positioning our assets, to be able to carry out different options - whatever options the president might choose."
Earlier, US defence officials said that navy commanders had decided to keep destroyer the USS Mahan in the area, even though it had been due to dock in Naples, Italy, and end its deployment. The ship carries Tomahawk missiles, which would be likely to be among the first weapons used in a US strike on Syria, and bolsters the navy's presence to four warships in the Mediterranean.
In an interview on Friday, President Barack Obama described the attacks on a rebel-held area in eastern Damascus that opposition activists claim killed 1,400 as "a big event of grave concern".
In the interview, President Obama struck a cautious note: "Sometimes what we've seen is that folks will call for immediate action, jumping into stuff, that does not turn out well, gets us mired in very difficult situations, can result in us being drawn into very expensive, difficult, costly interventions that actually breed more resentment in the region."
The New York Times reports that Obama aides were examining the 1999 Kosovo airstrikes as a precedent for using military force without a US mandate.