Washington warned on Wednesday that Syria was "spinning out of control" as Barack Obama telephoned Vladimir Putin to urge him to support efforts for political transition.
The US president told his Russian counterpart, who is Syria's key ally, to back a British-drafted resolution at the United Nations that would threaten Bashar al-Assad's regime with strong global sanctions.
Remarks by Putin's press secretary after the conversation suggested that the Kremlin may at last be edging away from Assad.
"As a whole the existing exchange of opinions shows that the appraisals of the situation in Syria and final goals of regulating [violence] for both sides coincide," said Dmitry Peskov. "Together with that, differences in approaches regarding the practical means of reaching a regulation [of violence] still exist," he added.
The White House said that, with the rebellion gaining strength, the "window is closing" for a peaceful solution.
"It is clear that the Assad regime is losing control of Syria," said Tommy Vietor, spokesperson for the National Security Council, adding that the sooner a workable transition takes place, "the greater the chance we have of averting a lengthy and bloody sectarian civil war".
Leon Panetta, the US defence secretary, said that the country was "rapidly spinning out of control", adding that the international community must "bring maximum pressure on Assad to do what is right, to step down and to allow for that peaceful transition".
Western leaders were careful not to condone the act of terror that killed three senior Syrian ministers, while arguing that it hastened the need for international unity. William Hague, the British Foreign Secretary, said: "This incident, which we condemn, confirms the urgent need for a Chapter VII resolution of the UN Security Council on Syria".
Laurent Fabius, the French foreign minister, said: "The French government has always condemned terrorism. That said, given the level of violence, this makes it even more necessary and urgent to find a political transition."
The West hopes that a unified resolution would provide a final push to the faltering regime and end the violence.
After meeting Putin on Tuesday, Kofi Annan, the international envoy to Syria, concluded that there was scope for compromise with Russia, which has blocked two UN resolutions so far.
Annan requested the delay of a UN vote on the resolution threatening tough sanctions yesterday, because, according to a Western diplomat, he thought "Putin wants unity at the Security Council".
