Syrian troops ousted rebels from the strategic town of Qusayr near the Lebanese border on Wednesday after a devastating 17-day assault led by Hezbollah fighters from across the frontier.
The government breakthrough came as France said the international community had to respond to test results from both French and British laboratories confirming the use of banned nerve agent sarin.
Washington’s initial response to the French declaration Tuesday was cautious, with a White House spokesman saying they needed to know more.
Then on Wednesday, US Secretary of State John Kerry said he had asked France to share its data.
Senior Russian, US and UN diplomats meeting in Geneva meanwhile, admitted that a widely anticipated international peace conference for Syria would not take place in June.
A date has still to be fixed for the talks to end the 26-month conflict, which monitors say has killed more than 94,000 people.
One of the main stumbling blocks was disagreement over who could take part.
The rebels conceded they had lost Qusayr after controlling it for a year, but opposition interim leader George Sabra declared they would fight on “until the whole country is liberated”.
The army said the “heroic victory” in the offensive, launched on May 19, served as a warning that it would “crush” the rebels and bring “security and stability to every inch of our land”.
The battle for Qusayr, a conduit for fighters and weapons just 10 kilometres (six miles) from Lebanon and linking Damascus to the Mediterranean coast, left the town in ruins.
Its capture opens the way for forces loyal to Syrian President Bashar al-Assad to move on the central city of Homs, much of which the rebels still control.
State television showed tanks rolling through deserted streets strewn with dust and bricks from shattered buildings, as well as cases of rockets which fleeing rebels had apparently abandoned in a hideout.
Soldiers looked on, their Kalashnikov assault rifles lowered, as bulldozers cleared away debris in the main square, where Syria’s flag flew atop a badly damaged clock tower.
The rebels admitted “this is a round that we have lost”, but added they would fight on against “the thousands of Lebanese mercenaries” — an apparent reference to Hezbollah.
The Syrian Observatory for Human Rights said the army and fighters from the powerful Shiite movement Hezbollah had taken Qusayr after an “intense bombardment” Tuesday night.
“The rebels have withdrawn… because they were short of ammunition,” it said.
At least 11 government troops had been killed and 25 wounded in the final hours of fighting for the town, it added.
Hours after Qusayr fell, at least five rockets launched from across Lebanon’s border with Syria hit the eastern Lebanese city of Baalbek, a Hezbollah stronghold.
Two landed in the city’s Roman ruins and the other three hit the city centre, a security source told AFP. Two people were injured in the attack.
British technicians have confirmed the findings of French laboratories that sarin gas has been used in the Syrian conflict.
French President Francois Hollande told reporters in Paris: “We have provided the elements of proof that now obligate the international community to act.”
On Tuesday, France said it had firm evidence that sarin had been used by the Syrian regime in at least one case.
Britain has also said it has evidence of sarin use, but added that it will wait for independent verification by the UN before drawing any conclusions about what, if any, action to take.
The UN Commission of Inquiry on Syria said there were “reasonable grounds” to believe both sides had used chemical weapons.
Kerry, on the sidelines of an Organization of American States meeting in Antigua on Wednesday, said he had spoken to French Foreign Minister Laurent Fabius about their findings.
“I asked him… whether he could send us the information that shows us the chain of custody of that evidence, so we know precisely where it came from,” he said.
Washington has always said that confirmation of battlefield use of such weapons would be a “game-changer” for Western policy on the conflict.
Russian, UN and US envoys meeting in Geneva failed to set a date for face-to-face talks between the Syrian regime and rebel representatives.
“It will not be possible to hold this conference in June,” UN-Arab League peace envoy Lakhdar Brahimi told reporters, adding that he hoped for a July date.
Russia and the US have struggled to overcome rebel objections to a proposed invitation to the government side without a prior commitment that Assad step down.
Russian Deputy Foreign Minister Mikhail Bogdanov said Iran’s participation, opposed by many Western and Arab governments but championed by Moscow, was also an obstacle.
“It’s a matter of principle because the whole composition of the conference should be balanced,” he said.
At a meeting in Cairo, Arab foreign ministers backed the Geneva peace initiative.
Ministers called for “efforts to persuade all the warring parties to allow logic and dialogue and negotiations to prevail in order to find a political solution between Syrians, as the only means”.