French President Francois Hollande has given a boost to US hopes of forging an international coalition for possible strikes against Syria after British MPs rejected any involvement in military action.
The White House had signalled on Thursday that President Barack Obama, guided by the "best interests" of the US, was ready to go it alone on Syria after deadly chemical weapons attacks last week.
But Russia, the Syrian regime's most powerful ally, warned any military strikes would "deal a serious blow to the entire system of world order".
UN arms experts on Friday began a final day of inspections of the sites of the suspected gas attacks before they leave the war-battered country on Saturday and report their findings to UN chief Ban Ki-moon.
Faced with an impasse at the UN Security Council and the British parliament's shock rejection of any punitive action against the Syrian regime, the United States has been forced to look elsewhere for international partners.
While Germany and Canada ruled out joining any military strikes, Hollande - whose country was a strident opponent of the war on Iraq - said the British vote would not affect his government's stance.
"France wants firm and proportionate action against the Damascus regime," Hollande said in an interview with Le Monde newspaper, hinting an attack was possible by Wednesday.
US Defence Secretary Chuck Hagel said the White House respected the British vote and that it was still seeking an "international coalition that will act together" against Syria's regime.
"We are continuing to consult with the British as with all of our allies. That consultation includes ways forward together on a response to this chemical weapons attack in Syria," he said in the Philippines.
The British rejection also came after the failure of an 11th-hour effort by British diplomats to win UN backing for action against President Bashar al-Assad's regime at a meeting of the permanent members of the Security Council.
"It is clear to me that the British parliament, reflecting the views of the British people, does not want to see British military action. I get that and the government will act accordingly," Cameron said.
His government was defeated by just 13 votes in its bid for a "strong humanitarian response" to Syria's alleged use of chemical weapons against its own people in the August 21 attacks.
Key Damascus allies Russia and Iran have warned against any Western intervention, saying it risked sparking a wider conflict in the already volatile Middle East.
But the military buildup was continuing in the region, while in Damascus the mood was heavy with fear and security forces were making preparations for possible air bombardments.
US warships armed with scores of cruise missiles are converging on the eastern Mediterranean, and US military officials have said they are ready to launch a powerful barrage against regime targets.
Envoys from the permanent Security Council members - Britain, China, France, Russia and the United States - met Thursday for the second time since Britain proposed a draft resolution to permit "all necessary measures" to protect Syrian civilians, but no breakthrough was reported.
Assad's ally and main arms supplier Russia has blocked all attempts to toughen sanctions against Damascus or authorise outside force to punish or unseat the regime.
Syria is in the 29th month of a vicious civil war in which more than 100,000 people have died and about three million more have become refugees or displaced, according to UN figures.
As the stand-off continued, the team of UN inspectors were in the final day of their investigations into the gas attacks that activists say killed more than 350 people, including women and children.
A UN spokesman said Thursday that the team had collected "considerable" evidence and will brief the UN secretary general soon after they leave Syria on Saturday.
Assad, whose regime strongly denies using chemical weapons and instead blames "terrorist" rebels, has remained defiant in the face of the threats.
"Syria will defend itself in the face of any aggression," he said on Thursday, vowing "victory" for his people.
