Russia accused the West on Wednesday of reneging on an agreement to establish a transitional government in Syria and of prolonging the bloodshed by encouraging the rebels fighting to topple President Bashar al-Assad.

Foreign Minister Sergei Lavrov said an agreement made by world powers and the then-peace envoy Kofi Annan in Geneva on June 30 was still valid and urged the West to do more to put it into practice.

"We remain convinced that what was achieved in Geneva should not be sabotaged. We will be demanding in the next few days a clear answer from our partners on whether they confirm what they signed in Geneva," Lavrov told a news conference.

"And if so, then why don't they take any measures to execute that plan?" he said, in the Belarus capital Minsk.

The Geneva deal did not specify what role, if any, Assad should have in a transitional administration that would seek to end the violence in an uprising that began in March 2011.

Since Geneva, fighting has intensified and Annan has resigned, his peace plan in tatters.

Most Western and Arab nations have called on Assad to go, saying his government's violent response to initially peaceful protests give him no place in a future Syria.

Russia, along with China, three times vetoed tougher U.N. sanctions against Damascus, a long-time strategic ally, but denies it is actively helping Assad to remain in power.

"It is essential that all external players put pressure on all Syrian sides and stop encouraging the opposition to continue its military struggle," Lavrov said.

U.S. State Department spokeswoman Victoria Nuland said the United States agreed with Lavrov's comments that the spirit of the Geneva declaration should "stay alive".

"We agree with that, which is precisely why the last time we went to the U.N. Security Council in June, our effort was to try to get a U.N. Security Council resolution that endorsed the Geneva understandings about transition, but put real teeth into a resolution in the form of sanctions if Assad didn't start moving in that direction," she said.

"And it was the Russians and the Chinese who chose to veto that."

U.N. human rights investigators said on Wednesday that both sides in Syria had committed war crimes but that rebel violations "did not reach the gravity, frequency and scale" of those carried out by the army and security forces.