"According to the South Ossetian information and press committee, the number of fatalities is estimated, according to preliminary information, at over 1,000," Teimuraz Kasayev said.
North Ossetia is part of Russia.
Georgia launched a major offensive early Friday morning using tanks, combat aircraft, heavy artillery and infantry.
Earlier Colonel Igor Konashenkov, an aide to the commander of the Russian Ground Forces, said about 10 Russian peacekeepers were killed and 30 wounded in the conflict zone.
The Russian Transportation Ministry's press service said Friday that Moscow would cut air links with Tbilisi.
Georgian military forces have begun retreating from the capital, Tskhinvali, the South Ossetian information and press committee said.
"Using grenade launchers [South] Ossetian local defense forces are destroying Georgian tanks. According to eyewitnesses they [the tanks] are on fire throughout the city," the committee said in a statement.
The statement also said that most of the city had been devastated by the Georgian military attack, which left the hospital destroyed and the republic's university on fire.