Russian President Dmitry Medvedev added his signature to a French-led peace plan already endorsed by Georgia and by leaders of its two rebel regions.
But Medvedev then ordered "extra security measures" in the conflict zone while foreign minister Sergei Lavrov said the withdrawal of Moscow's forces would depend on them being implemented.
The developments came in a day of rising tensions in the
war-torn region with Russia also threatening the arming of its Baltic fleet with nuclear warheads for the first time since the end of the Cold War.
A Russian defence source said: "The military is reviewing all its plans."
The move came in response to American plans for a missile defence shield in Europe and last week's agreement by Poland to station it on its soil.
Yesterday, Ukraine, a close ally of Georgia, inflamed the situation further by offering a Soviet-built satellite facility to the US as part of the defence system. Both Poland and the Ukraine may now be added to Russia's nuclear hit-list.
In the immediate crisis in Georgia, the peace agreement drafted last week authorises Russian forces to proceed with "additional security measures on a temporary basis" while awaiting the arrival of international peacekeepers – which requires passage of a UN Security Council resolution.
But Russia showed little sign yesterday of complying with US demands it should pull out of Georgia immediately.
US secretary of state Condoleezza Rice stepped up the pressure on Moscow by calling a meeting of foreign ministers from the 26 Nato states for Tuesday to consult allies about the implications of the Russian action.
Meanwhile, Kurt Volker, the US envoy to Nato, said the future of Nato-Russia dialogue and cooperation forums established after the Cold War would now be up for review.
US President George Bush, after meeting his national security team, said that there was progress in resolving the Georgia crisis. The fact that Russia and Georgia had signed a ceasefire agreement was "a hopeful step", but Russia needed to honour the agreement and withdraw its forces.
Bush, speaking at his Texas ranch, said the breakaway regions of South Ossetia and Abkhazia, which have been supported by Russia, were part of Georgia and "there's no room for debate on this matter".
Despite some reports of partial withdrawal from towns in Georgia yesterday, violence on the ground continued.
Georgia accused Russian troops of blowing up a railway bridge in broad daylight, saying the country's main east-west train link had been severed.
Russia's general staff denied attacking the bridge, declaring that hostilities that flared nine days ago around South Ossetia were over, as far as it was concerned.
"We are now in peace time. Why should we be blowing up bridges when our job is to restore?" said Colonel-General Anatoly Nogovitsyn, deputy chief of the general staff.
However, villagers said men in military uniform arrived by jeep, uncoiled wires and detonated a device remotely. The blast blew out windows of nearby homes. Irregular militias based in South Ossetia have also been operating against Georgian targets in recent days.
Meanwhile, rebels from the breakaway region of Abkhazia – backed by Russian forces – were also claimed by Georgia's Foreign Ministry to have seized control of 13 Georgian villages and a power plant bordering the region.
