Russia signed a peace deal to end the conflict in Georgia yesterday but said "extra security measures" were needed before a withdrawal could begin.
Russian President Dmitry Medvedev, working from the Black Sea resort of Sochi, added his signature to a French-led peace plan already endorsed by Georgia and by leaders of the two rebel regions at the heart of the conflict.
President George W. Bush, speaking in Texas, said it was a "hopeful step" but that Moscow must now pull its forces out.
Russian Foreign Minister Sergei Lavrov said withdrawal would depend on the 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" and blamed the difficult situation on the ground for delays.
The French-led agreement drafted last week authorises Russian forces to take extra security measures on a temporary basis pending the arrival of international peacekeepers - which requires a UN Security Council resolution.
Russian troops withdrew from an area 45 kilometres from the Georgian capital yesterday afternoon and a foreign military observer said it appeared a partial pullout might be under way, although that was not officially confirmed.
The United States demanded on Friday that Russia pull out of Georgia immediately, accusing Moscow of 'bullying' its tiny southern neighbour by sending in troops and tanks.
A simmering conflict between Georgia and Russia erupted into war nine days ago, when Tbilisi launched an assault to retake its separatist province of South Ossetia, prompting a huge counter-offensive from Moscow, which supports the rebels.
Violence on the ground continued.
Georgia accused Russian troops of severing the country's main east-west train link on Saturday by blowing up a railway bridge in broad daylight. Russia's General Staff denied attacking the bridge, saying it regarded hostilities as over.
One end of the bridge, near the town of Kaspi, lay collapsed on the riverbank in a pile of rubble and twisted steel, Reuters television pictures showed.
"We are now in peacetime. Why should we be blowing up bridges when our job is to restore?" Colonel-General Anatoly Nogovitsyn, deputy chief of the General Staff, told a daily official briefing in Moscow. "This therefore can only be yet another completely unverified statement."
A Reuters television crew interviewed villagers who said men in military uniform arrived by jeep, uncoiled wires and detonated a device remotely, blowing out nearby house windows.
The villagers blamed Russian forces but the identity of the attackers could not immediately be verified. Irregular militias, based in South Ossetia, have also been operating against Georgian targets in recent days.
Georgian President Mikheil Saakashvili signed the ceasefire on Friday after a five-hour meeting with visiting US Secretary of State Condoleezza Rice. Speaking afterwards, he denounced Russians as "21st century barbarians". British Foreign Minister David Miliband yesterday welcomed Medvedev's signing of the peace plan and said it must be "speedily and fully implemented", but denounced Russian "aggression" against Georgia and threats to neighbouring states.
Russian Colonel-General Nogovitsyn said Georgian snipers were still shooting in South Ossetia and that Russian forces had engaged a "Georgian sabotage group" near the Roki tunnel, the main crossing point for Russian troops into Georgia.