Georgian troops launched a major military offensive Friday to regain control over the breakaway province of South Ossetia, prompting a furious response from Russia _ which vowed retaliation and sent a column of tanks into the region.

The fighting was the worst outbreak of hostilities since the province won de-facto independence in a war that ended in 1992 _ raising fears that war could once again erupt.

Georgian Interior Ministry spokesman Shota Utiashvili said a convoy of Russian tanks had crossed into South Ossetia from the neighboring Russian province of North Ossetia and was moving toward the regional capital of Tskhinvali.

Russia's Channel 1 television earlier showed Russian tanks that it said had entered South Ossetia. The report said the convoy was expected to reach the provincial capital within a few hours.

Russian Prime Minister Vladimir Putin has warned that the Georgian attack will draw retaliation and the Defense Ministry pledged to protect South Ossetians, most of whom have Russian citizenship.

Georgian forces also shot down two Russian combat planes, according to Georgia's Interior Ministry spokesman, Shota Utiashvili. He said the planes were downed while they were raiding Georgian territory, but wouldn't give their type or any further details.

Russia's Defense Ministry denied an earlier Georgian report about one Russian plane downed. It had no immediate comment on the latest claim.

An Associated Press reporter saw tanks and other heavy weapons concentrating on the Russian side of the border with South Ossetia. Some villagers were fleeing into Russia.

"I saw them (the Georgians) shelling my village," said Maria, who gave only her first name. She said she and other villagers spent the night in a field and then fled toward the Russian border as the fighting escalated.

Separatist officials in South Ossetia said 15 civilians had been killed in fighting overnight. Georgian officials said seven civilians were wounded in bombing raids by Russia.

Putin, in Beijing to attend the Olympic opening ceremony, also said an unspecified number of the peacekeepers have been wounded.

Georgia declared a three-hour cease-fire to allow civilians to leave Tskhinvali. Georgia's Interior Ministry spokesman said troops were observing the cease-fire, which began at 3 p.m. local time (7 a.m. EDT).

A spokesman for President Bush said Russia and Georgia should cease hostilities and hold talks to end the conflict. NATO Secretary General Jaap de Hoop Scheffer said he is seriously concerned about the fighting and that the alliance is closely following the situation.

Georgia, which borders the Black Sea between Turkey and Russia, was ruled by Moscow for most of the two centuries preceding the breakup of the Soviet Union. The country has angered Russia by seeking NATO membership _ a bid Moscow regards as part of a Western effort to weaken its influence in the region.

Georgian President Mikhail Saakashvili long has pledged to restore Tbilisi's rule over South Ossetia and another breakaway province, Abkhazia. Both regions have run their own affairs without international recognition since splitting from Georgia in the early 1990s and built up ties with Moscow.

Relations between Georgia and Russia worsened notably this year as Georgia pushed to join NATO and Russia dispatched additional peacekeeper forces to Abkhazia.

South Ossetia officials said Georgia attacked with aircraft, armor and heavy artillery. Georgian troops fired missiles at Tskhinvali, an official said, and many buildings were on fire. The city's main hospital was among the buildings hit by Georgian shelling, the Russian news agency Interfax said.

The International Committee of the Red Cross said it is seeking to open a humanitarian corridor to guarantee safe access to Tskhinvali. Maia Kardova, ICRC spokeswoman in Tbilisi, said military vehicles are being given priority on the main road leading to the South Ossetia capital and this is making it difficult for rescue vehicles to get through.

Georgia's President said Russian aircraft bombed several Georgian villages and other civilian facilities.

"A full-scale aggression has been launched against Georgia," Saakashvili said in a televised statement.

He also announced a full military mobilization with reservists being called into action.

Seven civilians were wounded when three Russian Su-24 jet bombers flew into Georgia and bombed the town of Gori and the villages of Kareli and Variani, Deputy Interior Minister Eka Sguladze said at a briefing.

She said that four Russian jets later bombed Gori, the hometown of Soviet dictator Josef Stalin, but that raid didn't cause any casualties.

Saakashvili urged Russia to immediately stop bombing Georgian territory. "Georgia will not yield its territory or renounce its freedom," he said.

A senior Russian diplomat in charge of the South Ossetian conflict, Yuri Popov, dismissed the Georgian claims of Russian bombings as misinformation, the RIA-Novosti news agency reported.

Russia's Defense Ministry denounced the Georgian attack as a "dirty adventure." "Blood shed in South Ossetia will weigh on their conscience," the ministry said in a statement posted on its Web site.

"We will protect our peacekeepers and Russian citizens," it said without elaboration.

Russia's President Dmitry Medvedev later chaired a session of his Security Council in the Kremlin, vowing that Moscow will protect Russian citizens.

"In accordance with the constitution and federal law, I, as president of Russia, am obliged to protect lives and dignity of Russian citizens wherever they are located," Medvedev said, according to Russian news reports. "We won't allow the death of our compatriots go unpunished."

___

Associated Press writers Misha Dzhindzhikhashvili in Tbilisi, Vladimir Isachenkov in Moscow and John Heilprin at the United Nations contributed to this report.