The US has condemned the Syrian military's attack on Qusair, a strategic town over which it gained control after a bloody siege, BBC reported.
White House spokesman Jay Carney said pro-government forces had clearly needed help from their "partners in tyranny" - Hezbollah and Iran.
A BBC team that visited Qusair found that it was in ruins.
Meanwhile, France said growing proof of chemical weapons use in Syria "obliges the international community to act".
However, President Francois Hollande cautioned: "We can only act within the framework of international law".
He spoke hours after Syrian government forces retook full control of Qusair, after fierce fighting lasting some three weeks.
A team from the BBC were the first Western journalists to reach the city, and said they did not see a single building that had escaped damage.
More than 80,000 people have been killed in Syria and more than 1.5 million have fled the country since an uprising against President Bashar al-Assad began in 2011, according to UN estimates.
The UN and Arab League envoy to Syria, Lakhdar Brahimi, said the international conference might now be held in July, rather than June as had been planned.
He called the lack of agreement between Washington and Moscow "embarrassing", but also noted that neither side in the Syrian conflict was ready to commit to attending.