ORI, Georgia — Russia conducted airstrikes on Georgian targets on Friday evening, escalating the conflict in a separatist area of Georgia that is shaping into a test of the power and military reach of an emboldened Kremlin. Earlier in the day, Russian troops and armored vehicles had rolled into South Ossetia, supporting the breakaway region in its bitter conflict with Georgia.

The United States and other Western nations, joined by NATO, condemned the violence and demanded a cease-fire. Secretary of State Condoleezza Rice went a step further, calling on Russia to withdraw its forces. But the Russian soldiers remained, and Georgian officials reported at least one airstrike, on the Black Sea port of Poti, late on Friday night.

Before dawn on Saturday a senior Georgian official said by telephone that Russian bombers were flying over Georgia and that the presidential offices and residence in Tbilisi had been evacuated.

Neither side showed any indication of backing down. Prime Minister Vladimir V. Putin of Russia declared that “war has started,” and President Mikheil Saakashvili of Georgia accused Russia of a “well-planned invasion” and mobilized Georgia’s military reserves. There were signs as well of a cyberwarfare campaign against Georgia, as Georgian government Web sites were crashing intermittently during the day.

The escalation risked igniting a renewed and sustained conflict in the Caucasus region, an important conduit for the flow of oil from the Caspian Sea to world markets and an area where conflict has flared for years along Russia’s borders, most recently in Chechnya.

The military incursion into Georgia marked a fresh sign of Kremlin confidence and resolve, and also provided a test of the capacities of the Russian military, which Mr. Putin had tried to modernize and re-equip during his two presidential terms.

Frictions between Georgia and South Ossetia, which has declared de facto independence, have simmered for years, but intensified when Mr. Saakashvili came to power in Georgia and made national unification a centerpiece of his agenda. Mr. Saakashvili, a close American ally who has sought NATO membership for Georgia, is loathed at the Kremlin, in part because he had positioned himself as a spokesman for democracy movements and alignment with the West.

Earlier this year, Russia announced that it was expanding support for the separatist regions. Georgia labeled the new support an act of annexation.

The conflict in Georgia also appeared to suggest the limits of the power of President Dmitri A. Medvedev, Mr. Putin’s hand-picked successor. During the day, it was Mr. Putin’s stern statements from China, where he was visiting the opening of the Olympic Games, that appeared to define Russia’s position.

But Mr. Medvedev made a public statement as well, making it unclear who was directing Russia’s military operations. Officially, that authority rests with Mr. Medvedev, and foreign policy is outside Mr. Putin’s portfolio.

“The war in Ossetia instantly showed the idiocy of our state management,” said a commentator on the liberal radio station, Ekho Moskvy. “Who is in charge — Putin or Medvedev?”

The war between Georgia and South Ossetia, until recently labeled a “frozen conflict,” stretches back to the early 1990s, when South Ossetia and another separatist region, Abkhazia, gained de facto independence from Georgia after the collapse of the Soviet Union. The region settled into a tenuous peace monitored by Russian peacekeepers, but frictions with Georgia increased sharply in 2004, when Mr. Saakashvili was elected.

Reports conflicted throughout Friday about whether Georgian or Russian forces had won control of Tskhinvali, the capital of the mountainous rebel province. It was unclear late on Friday whether ground combat had taken place between Russian and Georgian soldiers, or had been limited to fighting between separatists and Georgian forces.

Marat Kulakhmetov, commander of Russian peacekeeping forces in Tskhinvali, said early on Saturday that South Ossetian separatists still held most of the city and that Georgian forces were only present on its southern edge. The newly arrived Russian Army forces had not entered the city, he said.

That report aligned with a statement by Georgia’s ambassador to the United Nations, Irakli Alasania, who said that Georgian military units held eight villages at the capital’s edge. Georgian officials asserted that Russian warplanes had attacked Georgian forces and civilians in Tskhinvali, and that airports in four Georgian cities outside the rebel area had been hit.

Shota Utiashvili, an official at the Georgian Interior Ministry, said they included the Vaziany military base outside of Tbilisi, the Georgian capital, a military base in Marneuli, and airports in the cities of Delisi and Kutaisi.

“We are under massive attack,” he said.

Late in the night, George Arveladze, an adviser to Mr. Saakashvili, said that Russian planes had bombed the commercial seaport of Poti, where one worker was missing and several others were wounded. Poti is an export point for oil from the Caspian Sea; Mr. Arveladze said the initial reports indicated that the oil terminal had not been struck.

Eduard Kokoity, the president of South Ossetia, said in a statement on a government Web site that hundreds of civilians had been killed in fighting in the capital. Russian peacekeepers stationed in South Ossetia said that 12 peacekeeping troops were killed Friday and that 50 were wounded. The claims of casualties by all sides could not be independently verified.

Analysts said that either Georgia or Russia could be trying to seize an opportune moment — with world leaders focused on the start of the 2008 Olympics in Beijing this week — to reclaim the territory, and to settle the dispute before a new presidential administration comes to office in the United States.

Richard C. Holbrooke, the former American ambassador to the United Nations, said that Russia’s aims were clear. “They have two goals,” he said. “To do a creeping annexation of South Ossetia and Abkhazia and, secondly, to overthrow Saakashvili, who is a tremendous thorn in their side.”

A spokesman for the Russian president, Mr. Medvedev, declined to comment.

The White House spokeswoman, Dana Perino, said the Bush administration had been talking to both sides to resolve the crisis. “We urge restraint on all sides — that violence would be curtailed and that direct dialogue could ensue in order to help resolve their differences,” she said.

The United States also said Friday that it would send an envoy to the region to try to broker an end to the fighting.

German Chancellor Angela Merkel issued a statement calling on both sides “to halt the use of force immediately.” Germany has taken a leading role in trying to ease tensions between Russia and Georgia over Abkhazia.

The trigger for the fresh escalation began last weekend, when South Ossetia accused Georgia of firing mortars into the enclave after six Georgian policemen were killed in the border area by a roadside bomb. As tensions grew, South Ossetia began sending women and children out of the enclave. The refugee crisis intensified Friday as relief groups said thousands of refugees, mostly women and children, were streaming across the border into the North Caucasus city of Vladikavkaz in Russia.

Early on Friday, Russia’s Channel One television showed Russian tanks entering South Ossetia and reported that two battalions reinforced by tanks and armored personnel carriers were approaching its capital. The Russian Defense Ministry said it was sending reinforcements to protect its peacekeepers there.

There were unconfirmed reports that Georgian forces had shot down two Russian planes and that its aircraft had bombed a convoy of Russian tanks. Russian state television showed what it said was a destroyed Georgian tank in Tskhinvali, its turret smoldering.

Women and children in Tskhinvali were hiding in basements while men had fled to the woods, said a woman reached by telephone in the neighboring Russian region of North Ossetia, who said she had been in phone contact with relatives there. She declined to give her name.

In Gori, a city outside South Ossetia and about 12 miles from Tskhinvali, residents said there had been sporadic bombing all day. The city was shaken by numerous vibrations from the impact of bombs on Friday evening. One Russian bomb exploded in Gori near a textile factory and a cellphone tower, leaving a crater.

At the United Nations on Friday, diplomats continued to wrangle over the text of a statement after attempts to agree to compromise language collapsed Friday afternoon, after nearly three hours of consultations.

The Russians, who had called the emergency session, proposed a short, three-paragraph statement that expressed concern about the escalating violence, and singled out Georgia and South Ossetia as needing to cease hostilities and return to the negotiating table.

But one phrase calling on all parties to “renounce the use of force” met with opposition, particularly from the United States, France and Britain. The three countries argued that the statement was unbalanced, one European diplomat said, because that language would have undermined Georgia’s ability to defend itself. Belgium, which holds the rotating presidency of the Security Council this month, circulated a revised draft calling for an immediate cessation of hostility and for “all parties” to return to the negotiating table. By dropping the specific reference to Georgia and South Ossetia, the compromise statement would also encompass Russia.

The Security Council was scheduled to meet Saturday to resume deliberations. China, in its statement during the early morning debate, had asked for a traditional cease-fire out of respect for the opening of the Olympics.

There are over 2,000 American citizens in Georgia, Pentagon officials said. Among them are about 130 trainers — mostly American military personnel but with about 30 Defense Department civilians — in the country assisting the Georgian military with preparations for deployments to Iraq.

The American military was taking no actions regarding the outbreak of violence, according to Pentagon and military officials. While there has been some contact with the Georgian authorities, the Defense Department had received no requests for assistance, the officials said.