There has been fierce fighting in the breakaway Georgian region of South Ossetia, where Georgian troops have been attempting to regain control from rebels.
Georgia said it moved in to "restore peace" in South Ossetia after recent clashes between its forces and separatist militias.
Russian troops have also entered the region and President Dmitry Medvedev has said that anyone who kills Russian citizens "will receive a deserved punishment".
Georgia has accused Russia of arming the separatists, who have been trying to break away since a civil war in the 1990s. Moscow denies the claim.
Georgian President Mikhail Saakashvili said that any involvement
by Russian forces in the conflict would result in "all out war".
Georgia has accused Russian jets of attacking airports and military bases, but Russia has denied any of its fighters have entered Georgian airspace.
Georgian troops have evacuated women and children from the regional capital Tskhinvali, where rebels claimed to be in control.
There are reports of many civilian deaths as well as military casualties. Russia says 10 of its peacekeeping troops have died.
Residents of Tskhinvali have been sheltering in basements as massive explosions rock the city and hospitals were reported to be struggling to cope with the numbers of injured.
People in the town of Gori, close to Tskhinvali, studied lists of the wounded to find the names of friends and family.
South Ossetian rebel leader Eduard Kokoity says Georgia has launched an all-out attack on Tskhinvali, in what he calls "a perfidious and base step".