The United States condemned an assault by Syrian troops on the town of Qusayr, claiming the regime had relied on Hezbollah to win the battle and caused tremendous suffering.
Syrian troops ousted rebels from the strategic town near the Lebanese border early Wednesday after a devastating 17-day assault led by Hezbollah fighters from across the border.
The government breakthrough came a day after France said the international community had to respond to test results from both French and British laboratories confirming Syria's use of the banned nerve agent sarin.
US Secretary of State John Kerry said he had asked France to share its data.
In the meantime, a White House statement said: "The United States condemns in the strongest possible terms the Assad regime's assault on Qusayr, which has killed untold numbers of civilians and is causing tremendous humanitarian suffering.
"It is clear that the regime could not contest the opposition's control of Qusayr on its own, and is depending upon Hezbollah and Iran to do its work for it in Qusayr."
White House spokesman Jay Carney renewed a call for Hezbollah and Iran to withdraw their fighters from Syria.
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.
