President Dmitry Medvedev added his signature to a French peace plan already endorsed by Georgia and by South Ossetia and Abkhazia.
Russian Foreign Minister Sergei Lavrov said withdrawal would depend on extra security measures ordered by Medvedev, the nature of which was not made clear.
Asked how long the withdrawal might take, Lavrov told reporters: "This does not just depend on us." He blamed the difficult situation on the ground for delays.
Despite the peace deal, a gulf still separates Moscow and Tbilisi over the future of the rebel regions, which broke away from Georgia in the early 1990s and declared independence, albeit without international recognition.
U.S. President George Bush echoed the Georgian position on Saturday, saying South Ossetia and Abkhazia were part of Georgia "and there's no room for debate on this matter."
But Lavrov has already told the West to "forget about Georgia's territorial integrity," saying the facts on the ground dictate a different reality.
Russia says 1,600 civilians died when Georgia stormed South Ossetia, though the figure has not been independently verified. Georgia says at least 175 people have been killed and hundreds more injured. The figure does not include South Ossetia.