The US has condemned the assault on Qusayr by Syrian government forces as celebrations erupted over the routing of rebels who have been battling for control of the southwestern city.
Washington accused the government of President Bashar al-Assad of having relied on Hezbollah fighters from neighbouring Lebanon, backed by Iran, to win the battle for the strategic area.
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"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," Jay Carney, the White House spokesman, said in a statement.
"It is clear that the regime could not contest the opposition's control of Qusair on its own, and is depending upon Hezbollah and Iran to do its work for it in Qusayr."
On Wednesday, Iran congratulated the Syrian government on its victory, with Foreign Minister Hossen Amir Abdolahia calling the rebels "terrorists".
Syrian troops routed the rebels in Qasayr on Wednesday following a devastating 17-day assault that had left many civilians trapped in the town with no access to water and electricity.
The battle for the city, a conduit for fighters and weapons just 10km from Lebanon and linking Damascus to the Mediterranean coast, left the town in ruins.
Its capture opens the way for forces loyal to Assad to move on the central city of Homs, much of which the rebels still control.
The Arab League, which has given Syria's seat to the opposition, also condemned the assault, issuing a statement in the Egyptian capital Cairo denouncing Hezbollah.
Carney renewed a call for Hezbollah and Iran to withdraw their fighters from Syria.
Fighting has continued despite the rebels' defeat in Qusayr. On Thursday the rebels briefly seized the only crossing along the Israel-Syria ceasefire line in the Golan Heights, before regime forces recaptured it, an AFP correspondent and Israeli sources said.
Several pro-Hezbollah towns in Lebanon's Baalbek region were also hit by rockets overnight.
In the Lebanese town of Tripoli, Salafi leaders threatened to attack Alawite neighbourhoods if the army did not deploy in the city.
Rebels to fight on
Meanwhile, France, one of the UN Security Council members pushing for intervention in the Syrian conflict, has said it has proof Syria used deadly sarin gas - a banned nerve agent - on at least one occasion, and French President Francois Hollande kept up the pressure Wednesday.
"We have provided the elements of proof that now obligate the international community to act," he told reporters in Paris.
|Analysis: The Fall of Qusayr
Britain said it also had evidence of sarin use, but had passed it on to the UN for independent verification a week ago and would wait for its findings.
In Syria, the rebels conceded the loss of Qusayr after controlling it for a year, but George Sabra, interim leader of the opposition, 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 Syrian conflict began as an uprising in March 2011 and has since degenerated into a civil war, with the potential to suck in neighouring countries.
The UN estimates at least 70,000 have been killed in the conflict while hundreds of thousands have fled to Lebanon, Turkey and Iraq.