The condemnation came as efforts to organise a peace conference in Geneva between the warring sides stalled, with diplomats blaming differences over who could take part.
Washington accused the regime of President Bashar al-Assad of having relied on Hezbollah fighters from Lebanon to win the battle for the strategic town of Qusayr near the Lebanese border, after rebels were ousted following a devastating 17-day assault.
A White House statement condemned the Syrian regime's assault on the town, "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," the statement said.
White House spokesman Jay Carney renewed a call for Hezbollah and Iran to withdraw their fighters from Syria.
France 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.
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.
The UN Commission of Inquiry on Syria has said there are "reasonable grounds" to believe both sides had used chemical weapons.
Kerry, on the sidelines of an Organization of American States meeting in Antigua, 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.
In Syria, the rebels conceded the loss of 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 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.
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.
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.
Human Rights Watch accused both sides of using schools for military purposes in a report released early Thursday.
It described how pro-regime forces fired on school buildings that were not being used for military purposes.
One student described hiding under her desk as machinegun fire hit the school walls; another told how she had fled as tank shells hit her school.
"Syrian children have had to face things in the horrors of war that no child should have to bear: interrogated, targeted, and attacked," said Priyanka Motaparthy, who compiled the report.
AFP