Following marathon talks in Munich, the powers, including the United States, Russia and more than a dozen other nations, reaffirmed their commitment to a political transition when conditions on the ground improved.

But diplomats cautioned that Russia had until now not demonstrated any interest in seeing President Bashar al-Assad replaced and was pushing for a military victory.

At a news conference, US Secretary of State John Kerry acknowledged the Munich meeting produced commitments on paper only.

"What we need to see in the next few days are actions on the ground, in the field," he said, adding that "without a political transition, it is not possible to achieve peace."

Kerry said the cessation of hostilities would not apply to Islamic State and other militant groups fighting in Syria, he added. Islamic State militants control large parts of Syria and Iraq.

Russian Foreign Minister Sergei Lavrov told the news conference that Russia would not stop air attacks in Syria, saying the cessation of hostilities did not apply to Islamic State and al Nusrah, which is affiliated with al Qaeda.

The United States and European allies say few Russian strikes have targeted those groups, with the vast majority hitting Western-backed opposition groups.

Lavrov said peace talks should resume in Geneva as soon as possible and that all Syrian opposition groups should participate. He added that halting hostilities would be a difficult task.

But British Foreign Secretary Philip Hammond said ending fighting could only succeed if Russia stopped air strikes supporting Syrian government forces' advance against the opposition.

It cautioned, however, that the agreement must prove to be effective before it joins political talks with government representatives in Geneva.