GORI, 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.

Russian military units — including tank, artillery and reconnaissance — arrived in Tskhinvali, the capital of South Ossetia, on Saturday to help Russian peacekeepers there, in response to overnight shelling by Georgian forces, state television in Russia reported, citing the Ministry of Defense. Ground assault aircraft were also mobilized, the Ministry said.

Also 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. The official added that Georgian forces still had control of Tskhinvali.

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, 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.

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 had been hit.