US President Barack Obama was meeting top national security aides on Friday over possible missile strikes to punish Syria for its alleged use of chemical weapons in a deadly attack last week.
France gave its backing to the US plans after British lawmakers voted against any involvement in military action against Damascus and other close allies including Germany said they would not sign up.
The White House has signalled that Obama, guided by the "best interests" of the United States, was ready to go it alone on Syria after accusing President Bashar al-Assad's regime of using chemical weapons against its own people.
But Russia, Syria's most powerful ally, questioned US intelligence on the August 21 gas attacks and warned against any military strikes without UN backing.
In Damascus, UN experts carried out their final investigations into the attacks east of the capital that activists say killed more than 300 people, visiting a hospital where victims were reportedly being treated.
The team is due to leave the war-battered country Saturday and report back immediately to UN chief Ban Ki-moon, who has appealed to the West to allow time for their findings to be assessed.
Ban is due to meet ambassadors from the five UN Security Council permanent members at 1600 GMT, while US Secretary of State John Kerry will make a statement on the Syria crisis at 1630 GMT.
Faced with an impasse at the Security Council and the British parliament's shock vote on Thursday, Obama has been forced to look elsewhere for international partners.
While Germany and Canada ruled out joining any military strikes, French President Francois Hollande -- whose country was a strident opponent of the US-led war on Iraq -- said the British decision 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.
US Defence Secretary Chuck Hagel said the White House was still seeking an "international coalition that will act together" against Assad's regime.
The British government's defeat in parliament came after the failure of an 11th-hour effort by British diplomats to win UN backing for action at a meeting of Security Council permanent members.
Gruesome pictures of some of the reported victims of the attacks, including children, shocked the world and piled on the pressure for a response that could draw a reluctant West into the vicious Syrian civil war.
But Russia and Iran, and even some US allies and respected commentators, have warned against any intervention, saying it risked sparking a wider conflict.
Divisions over Syria have further chilled the frosty relations between Washington and Moscow ahead of the G20 summit next week in Saint Petersburg, where pointedly there will be no face-to-face talks between Obama and Russian President Vladimir Putin.
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US National Security Council spokeswoman Caitlin Hayden said Obama's decision-making "will be guided by what is in the best interests of the United States".
"He believes that there are core interests at stake for the United States and that countries who violate international norms regarding chemical weapons need to be held accountable."
Earlier in the week, reports had suggested a Western strike was imminent, but questions have been raised about the quality of the intelligence linking Assad to the gas attack.
Syria's Foreign Minister Walid Muallem reportedly told the UN chief that Damascus would reject any "partial" conclusions about the attack before full analyses are undertaken.
The military buildup was continuing in the region, with US warships armed with scores of cruise missiles converging on the eastern Mediterranean.
In Damascus the mood was heavy with fear and security forces were making preparations for possible air bombardments, pulling soldiers back from potential targets.
More than 100,000 people have died since the conflict erupted in March 2011 and two million are refugees, half of them children, according to the United Nations.
Some commentators have questioned the wisdom of Obama dragging the United States into another conflict after the wars in Iraq and Afghanistan -- particularly as Al-Qaeda militants are among the rebels fighting the Assad regime.
Some members of the US Congress voiced support for limited surgical missile strikes, while urging transparency from the administration.
But an opinion poll released Friday said half of all Americans believe Obama should not intervene, and Italian Foreign Minister Emma Bonino said military strikes risked escalating the conflict into a "global conflagration".
Syria's main arms supplier Russia has blocked all attempts to toughen sanctions against Damascus or authorise outside force to punish or unseat the regime.
Putin's chief foreign policy aide Uri Ushakov complained Friday that the US was not sharing its intelligence about the gas attacks and so "we do not believe it".
And he said any military action that bypasses the Security Council "will deal a serious blow to the entire system of world order".
Assad, whose regime strongly denies using chemical weapons and instead blames "terrorist" rebels, remains defiant.
"Syria will defend itself in the face of any aggression," he said Thursday, vowing "victory" for his people.
Syria's jittery neighbours have also taken steps to boost security, while several airlines have changed their times for flights into Beirut, with Lebanon always vulnerable to spillovers of the conflict in its larger neighbour.