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 said.

"And if so, then why don't they take any measures to execute that plan?" he told a news conference 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 has opposed tougher U.N. sanctions against Damascus, a long-time strategic ally, but denies it is actively helping Assad 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.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.