Charles Heyman, a former British officer who edits The Armed Forces of the UK, said the lack of a U.N. Security Council resolution authorizing the use of force against the Syrian government greatly complicates matters for the West. He said that may make it difficult for Cameron to win parliamentary backing.
"It's clear the governments want some form of military operation, but if the Security Council doesn't recommend it, then the consensus is that it's plainly illegal under international law," Heyman said. "The only legal way to go to war is in self-defense and that claim is difficult to make."
Russia, a permanent member of the Security Council, has steadfastly opposed any international action against Syria.
Italian Foreign Minister Emma Bonino said her country would not back any military action against Syria unless it was authorized by the Security Council -- even though it considers a chemical attack to be a war crime.
German Foreign Minister Guido Westerwelle said Monday that if the Syrian government were proven to have been behind the gas attack, then Germany would support "consequences." But with less than four weeks until national elections, it is unlikely Germany would commit any forces.
Center-left opposition parties have rejected military intervention without U.N. proof that the Syrian government was behind the attack. And a senior member of Chancellor Angela Merkel's party said the German military was already at "the breaking point" due to commitments in Afghanistan and elsewhere.
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.
In supporting calls for action against Syria, the 22-member Arab League, which is dominated by Gulf powerhouses Saudi Arabia and Qatar, provides indirect Arab cover for any potential military attack by Western powers.
At an emergency meeting, the Arab League also urged members of the Security Council to overcome their differences and agree on "deterrent" measures.
"The council holds the Syrian regime totally responsible for this heinous crime and calls for all involved in the despicable crime to be given a fair international trial like other war criminals," the Arab League said in a statement.
Heyman predicted a possible three-phase campaign, with the first step -- the encirclement of Syria by Western military assets by air and sea -- already underway.
"Phase two would be a punitive strike, taking out high-value command and control targets and communications centers," Heyman said. "That could be done easily with cruise missiles from ships and aircraft. Phase three would be a massive takedown of Syrian air defenses. That would have to be done before you could take out artillery and armor, which is the key to long-term success."
Gregory Katz reported from London. Also contributing were Associated Press writers Zeina Karam and Bassem Mroue in Beirut, John Heilprin in Geneva, Julie Pace and Matthew Lee in Washington, Lori Hinnant in Paris, Lynn Berry in Moscow, Menelaos Hadjicostis in Cyprus, Sarah El Deeb in Cairo and Charmaine Noronha in Toronto.