President Obama is facing huge pressure on Syria after regime forces apparently crossed his famous "red line" by using chemical weapons.
After Russia and China again watered down the United Nations response to the deaths, France led the charge, saying force would be used in response.
Laurent Fabius, the country's foreign minister, hinted a decision would have to be made "in other ways" if the Security Council could not agree.
And Britain told President Assad's sympathisers to "wake up" to what was going on.
Images of children convulsing and foaming at the mouth have shocked the world as opposition groups claimed 1,400 people had been killed.
Although they cannot be verified, a UN inspection team has arrived in the country, and Britain says it would be a "shocking escalation" if they find the damning evidence.
Senior figures said the latest reports called into question Obama's "red line" pledge, made almost exactly a year ago.
And opposition representatives insisted the line had clearly been crossed.
A year ago, Barack Obama set out a "red line" that chemical weapons would change his thinking on the Syrian crisis.
“A red line for us is (if) we see a whole bunch of chemical weapons moving around, or being utilized. That would change my calculus,” he said.
Analysts said the White House had altered its rhetoric to avoid being pinned to the quote.
On Wednesday, spokesman Josh Earnest said: "The United States is deeply concerned by reports that hundreds of Syrian civilians have been killed in an attack by Syrian government forces, including by the use of chemical weapons."
The apparent gas attack, in rebel-held parts of eastern Damascus, is thought to be the most significant use of chemical weapons since thousands of Kurds were gassed by Saddam Hussein in Halabja 25 years ago.
Opposition activists said men, women and children were killed as they slept.
The Syrian opposition said President Bashar al-Assad's forces fired rockets that released deadly fumes over rebel-held eastern Damascus suburbs, which are part of what is known as the Ghouta.
Former British diplomat Jeremy Greenstock told the BBC's Today programme: "The Syrian regime is getting further and further into illegitimacy as far as the future is concerned."
Syrian Information Minister Omran Zoabi said the allegations were "illogical and fabricated."
The Syrian government acknowledged it had launched a major offensive in rebel-held districts in the east of the capital – described by pro-regime media as the biggest since the start of the civil war.
Syrian activists inspect the bodies of people they say were killed by nerve gas
But officials strongly denied using chemical weapons, with Assad's officials insisting they would never use poison gas against Syrians.
Russia, which has supported the regime and vetoed past attempts to secure a tough UN resolution, backed up Syrian government denials by saying it looked like a rebel "provocation" to discredit Assad.
However, George Sabra, the head of the main Syrian opposition group, laid the blame squarely at the Assad regime, saying the scenes "constitute a turning point in the regime's operations".
"This time it was for annihilation, rather than terror," he said.
The apparent attack comes just days after a 20-strong team of UN weapons inspectors arrived in the capital to investigate whether chemical weapons have been used in the conflict.
Shocking images showed scores of bodies - some of them small children - laid on the floor of a clinic with no visible signs of injury. Some showed people with foam around their mouths.
Following an emergency meeting of the UN Security Council, Deputy Secretary-General Jan Eliasson, said inspectors are hoping to conduct an investigation into whether chemical weapons have been used in the conflict.
British Foreign Secretary William Hague urged international supporters of the Syrian regime to "wake up to...its murderous and barbaric nature" ahead of the UN meeting.