The White House has said it is "highly likely" that it is President Bashar al-Assad's regime, not rebels, behind any chemical weapons use.
Earlier, a leading member of the Independent International Commission of Inquiry on Syria, told Swiss TV statements from victims of the conflict suggested rebels had used the nerve agent Sarin.
Carla Del Ponte said that there was "strong, concrete suspicions but not yet incontrovertible proof".
However, she did not rule out the possibility that they might also have been used by government forces.
An Israeli airstrike on a military complex near Damascus killed at least 42 Syrian soldiers, the Syrian Observatory for Human Rights said.
Russia has said it is concerned the chance of foreign military intervention in Syria is growing, following reports of Israeli airstrikes around Damascus.
"We are seriously concerned by the signs of preparation of global public opinion for possible armed intervention in the long-running internal conflict in Syria," Russian Foreign Ministry spokesman Alexander Lukashevich said in a statement.
Fawaz Gerges, Director of the Middle East Center at the London School of Economics, has told ITV Daybreak that "it would take a spark to ignite the conflict in Syria."
He added: "The conflict in Syria has mutated and taken a very dangerous turn. It has major implications for regional stability and international peace and security."
The deputy foreign minister of Syria, Faisal Al Mekdad, said Israeli air strikes against targets around Damascus amounted to a "declaration of war" and threatened to retaliate. Speaking to CNN he said:
"This is a declaration of war, this is not something that is strange, but we dealt with this on several occasions and we retaliated the way we want, and the retaliation was always painful to Israel."
Middle East Correspondent John Ray reports on the diplomatic shock waves created by the air strikes against Damascus.
The United States was not given any prior warning before air strikes in Syria against what officials say were weapons headed for Hezbollah militants, a US intelligence official has said.
Without confirming that Israel was behind the attacks, the intelligence official said that the United States was essentially told of the air raids "after the fact" and was notified as the bombs went off. Speaking anonymously to Reuters the official said:
It would not be unusual for them to take aggressive steps when there was some chance that some sophisticated weapons system would fall into the hands of people like Hezbollah
Syria claimed Israel's attack on a military facility north of Damascus is a declaration of war.
The deputy foreign minister claimed Israelis were siding with Islamic terrorists and its aggression "opens the door to all possibilities".
Israel has not officially confirmed or denied carrying out the air strikes overnight.
There is "grave concern" over reports of Israel air strikes over Syria, the United Nations Secretary-General Ban Ki-moon has said today.
A spokesperson for the UN Secretary-General said: "At this time, the United Nations does not have details of the reported incidents. Nor is the United Nations in a position to independently verify what has occurred.
"The Secretary-General calls on all sides to exercise maximum calm and restraint, and to act with a sense of responsibility to prevent an escalation of what is already a devastating and highly dangerous conflict".
Two US senators said Syria's air defence system isn't as strong as was once thought and add that recent air strikes show the regime is vulnerable.
Former Republican presidential candidate Senator John McCain and Democratic Senator Patrick Leahy said today that Syrian President Bashar al-Assad's air defence systems were not impenetrable.
In recent days reported air strikes launched by Israel targeted a shipment of advanced missiles believed to be headed for the Lebanese militant group Hezbollah.
Speaking on Fox News Sunday, McCain said Syria had crossed another "red line" in transferring weapons to US aggressors, but he does not believe US ground forces should go into Syria to secure the weapons.
Leahy spoke to NBC's "Meet the Press".