BBC News: Georgia says its forces have withdrawn from the separatist enclave of South Ossetia, and that Russian troops are now in control of the regional capital.
An interior ministry spokesman told the BBC it was not a military defeat but a necessary step to protect civilians from a "humanitarian catastrophe".
Georgia says Russia has brought an additional 10,000 soldiers across its frontiers, readying for a raid.
Earlier, Russian jets bombed a military airfield close to the Georgian capital.
There was no independent confirmation of the attack, although the BBC's Gabriel Gatehouse, who was in Tbilisi, said he had heard a loud explosion about the same time.
Georgian troops have pulled back to positions at or south of those held on 6 August, when the current hostilities began, said Georgian Interior Ministry spokesman Shota Utiashvili.
He told the BBC that the withdrawal was necessary because of the mass casualties both within Georgia and South Ossetia, at the hands of the Russians.
Mr Utiashvili said 100 Georgian soldiers had been killed and many more injured.
Earlier, Georgia's President Mikhail Saakashvili called for an immediate ceasefire to stop what he described as an "annihilation" of his country's democracy.
Based on Russian and South Ossetian estimates, the death toll on the South Ossetian side was at least 1,500. According to Moscow, all but a few of the dead were civilians.
Russian Prime Minister Vladimir Putin has accused Georgia of genocide against the South Ossetian people and defended Moscow's military action to intervene directly.
On Saturday, he flew to the Russian city of Vladikavkaz, close to the border with South Ossetia, where he met those who had fled the violence. He said the territorial integrity of Georgia had "suffered a fatal blow", suggesting that it was unlikely that South Ossetia would re-integrate with the rest of Georgia after the conflict.
He said the conflict had created at least 34,000 refugees.
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