The collapse of support puts pressure on President Barack Obama as resistance to the mission grows at home — and comes with the irony that Paris was the most vocal critic of the U.S.-led invasion of Iraq.
French President Francois Hollande pledged backing Friday for Obama's plans to hit the Damascus regime.
"The chemical massacre of Damascus cannot and must not remain unpunished," Hollande said in an interview with the newspaper Le Monde, published Friday, as U.N. experts in Damascus began what is expected to be the last day of their probe into the alleged attack.
U.S. Defense Secretary Chuck Hagel, speaking from Manila, Philippines, issued an impassioned defense of the principles behind the planned strike.
"I don't know of any responsible government around the world ... that has not spoken out in violent opposition to the use of chemical weapons on innocent people," said Hagel, adding that such attacks violate basic standards of decency.
He said that Washington would continue to seek partners in its Syria mission: "Our approach is to continue to find an international coalition that will act together."
The U.S. administration shared intelligence with lawmakers Thursday aimed at convincing them the Syrian government used chemical weapons against its people and must be punished.
"The president of the United States is elected with the duty to protect the national security interests in the United States of America," said White House spokesman Josh Earnest.
Meanwhile, in Damascus, shops and supermarkets filled with Syrians stocking up on bread, canned food and other necessities ahead of the expected U.S. strikes, although there appeared to be no signs of panic or food shortages. Prices have shot up because of the high demand, residents complained.
Kheireddine Nahleh, a 53-year-old government employee, said he was not particularly worried about the U.S. threats. "We got used to the sound of shelling ... Death is the same be it with a mortar or with an American missile," he said. "I'm not afraid."
On the last expected day of chemical weapons inspections, three U.N. vehicles headed out for more on-site visits, following an early morning delay.
The U.N. said Thursday that the inspectors would wrap up their investigation Friday and leave Syria for the Hague, Netherlands, the following day. Some of the experts will travel to laboratories in Europe to deliver the material they've collected this week during trips to the Damascus suburbs purportedly hit by toxic gas.
Russia, which as a firm backer of the Assad regime is fiercely hostile to military intervention, expressed bewilderment Friday at why the U.N. team was leaving so soon.
"We don't quite understand why the entire team had to be going back to the Hague when there are many questions about a possible use of chemical weapons in other areas in Syria," said Yuri Ushakov, President Vladimir Putin's foreign policy adviser.
Russian Foreign Minister Sergey Lavrov has warned that military strikes would lead to long-term destabilization of Syria and the region. He has spoken against any use of force without U.N. Security Council approval, which he said would be a "crude violation of international law." Russia has remained a strong ally of Syria throughout the civil war, which has left more than 100,000 people dead.
In Paris, Hollande suggested that action could even come ahead of Wednesday's extraordinary session of the French Parliament, called to discuss the Syria situation; lawmakers' approval is not needed for Hollande to order military action.
"I will not take a decision before having all the elements that would justify it," he told Le Monde. However, noting that he had convened parliament, he added: "And if I have (already) committed France, the government will inform (lawmakers) of the means and objectives."
The British parliament voted late Thursday against military action in Syria, whittling down the core of the planned coalition to the United States and France. Italy and Germany have said they won't take part in any military action.
A spokesman for German Chancellor Angela Merkel said Friday that there's been no request for a military commitment by his country and the government is not planning any. "We have not considered it and we are not considering it," Stephen Seibert said.
Hollande said that France is among the few nations capable of "inflicting a sanction by the appropriate means" and "it is ready." But a decision will be made in close coordination with allies, he added.
France has historic ties to Syria, having once ruled the country; it also has warplanes and strategic interest in the region. Paris has embraced the Syrian opposition and urged a firm response against Assad over the purported Aug. 21 chemical weapons attack outside Damascus.
French military analysts say France's most likely role would be from the air, including use of Scalp cruise missiles that have a range of about 500 kilometers (300 miles), fired from Mirage and Rafale fighter jets. French fighters could likely fly directly from mainland France — much as they did at the start of a military campaign against Islamic radicals in Mali earlier this year — with support from refueling aircraft. France also has six Rafale jets at Al Dhafra air base, near Abu Dhabi in the United Arab Emirates on the Persian Gulf, and 7 Mirage-2000 jets at an air base in Djibouti, on the Red Sea.
Hollande reiterated that any action is aimed at punishing Assad, not toppling him.
"I won't talk of war but of a sanction for a monstrous violation of the human person," he said. "It will have a dissuasive value."