Russia's inaction over the crisis in Syria is keeping President Bashar al-Assad in power, U.S. Secretary of State Hillary Clinton has said.
Russia's "refusal to join us in some kind of constructive action is keeping Assad in power, well-armed, able to ignore the demands of his own people, the region and the world," Clinton said at the U.S. Naval Academy on Tuesday.
She said the U.S. would again try to persuade Russia to support UN action that would at least allow humanitarian aid when G8 foreign ministers meet in Washington on Wednesday.
Clinton also said the "likelihood of regional conflict and civil war is increasing" in Syria.
Russia is the largest supplier of arms to Syria and has a naval base there.
Syrian opposition activists say more than 800 people have been killed since Assad accepted UN envoy Kofi Annan's peace plan late last month.
Clinton's comments came as Syrian security forces continued military operations despite an agreement to withdraw troops by Tuesday.
However, Annan said earlier on Tuesday his efforts to end the unrest were still "very much alive."
Syrian Foreign Minister Walid al-Muallem said on Tuesday in Moscow that army units had begun withdrawing from some towns and cities, a claim denounced by a French foreign ministry spokesman as an "unacceptable lie."
"Every effort must be made to achieve a cessation of violence in all its forms on 12 April at 06:00 [03:00 GMT]," Annan said in a letter to the UN Security Council.
"There is no more time to lose," Annan said. "We must all push for an end to the bloodshed before Syria plunges into the abyss."