Support for some sort of international military response is likely to grow if it is confirmed that Assad's regime was responsible.
The U.N confirmed its chemical weapons team's mission faced a one-day delay Tuesday to improve preparedness and safety after unidentified snipers opened fire on the team's convoy Monday.
In Geneva, U.N. spokeswoman Alessandra Vellucci said the U.N. inspection team might need longer than the planned 14 days to complete its work. She said its goal is to determine what chemical weapons might have been used in the Aug. 21 attack.
The Obama administration is making a legal argument for undertaking a military response to the use of chemical weapons against civilians in Syria, but said any action against the Syrian regime is not intended to depose Assad.
Carney said the United States and 188 other nations are signatories to a chemical weapons convention opposing the use of such weapons. Those countries have a stake in ensuring that international norms must be respected and there must be a response to a clear violation of those norms, he said.
In a veiled allusion to difficulties in getting any strong action through the Security Council, France's Hollande said that "international law must evolve with the times. It cannot be a pretext to allow mass massacres to be perpetrated."
He then went on to invoke France's recognition of "the responsibility to protect civilian populations" that the U.N. General Assembly approved in 2005.
Obama discussed Syria on Tuesday with Prime Minister Stephen Harper of Canada, a NATO ally, and in recent days with Cameron, Hollande and Australian Prime Minister Kevin Rudd.
Harper's office said he agreed with the assessment that the Assad regime used chemical weapons against its own people, and called it an outrage that requires a "firm response," without defining what that might entail.