Some 1,400 people are reported dead in fighting involving Georgian and Russian forces in Georgia's breakaway region of South Ossetia.
Both sides accuse the other of being the aggressor.
Georgian President Mikhail Saakashvili said Friday he ordered an all-out offensive to regain control of South Ossetia, after Russian tanks entered the region. He accused Russia of waging war on Georgia.
Russia said its troops were responding to a Georgian assault to retake South Ossetia.
A Georgian Interior Ministry spokesman, Shota Utiashvili, said Russian shelling of the South Ossetian capital, Tskhinvali, forced Georgian troops from part of the city, which the Georgians captured earlier in the day.
Russian authorities reported at least 12 Russian peacekeepers killed and at least 50 wounded in the fighting. South Ossetian officials said the number of civilian dead exceeded 1,000.
Georgian officials demanded that Russia stop air attacks on Georgian cities, attacks that Russia denies carrying out. Georgia said Russian planes bombed the port city of Poti and at least two Georgian air bases outside Tbilisi.
Georgia's Ambassador to the United Nations, Irakli Alasania, told the U.N. Security Council that Russian bombers also are attacking parts of Abkhazia, Georgia's other breakaway region. He said Georgian government buildings in Tblisi are being evacuated.
A top Georgian official said President Saakashvili is planning to declare martial law within hours, a move that would give him a free hand to manage the conflict. The country is moving half of the 2,000 troops it has in Iraq to South Ossetia, to help in the fighting.
South Ossetia declared independence from Georgia in the early 1990s, sparking fighting and the dispatch of Russian peacekeepers. Georgia accuses the peacekeepers of backing the separatists and has vowed to bring South Ossetia back under central government control.
Some information for this report was provided by AFP, AP and Reuters.