GORI, Georgia — Russian troops entered a breakaway region of Georgia on Friday after Georgian forces pushed into the capital of the pro-Russian enclave, in a sharp escalation of the longstanding conflict.

Russian Prime Minister Vladimir V. Putin declared that “war has started” and President Mikheil Saakashvili of Georgia accused Russia of a “well-planned invasion," saying he had mobilized Georgia’s military reserves.

Reports conflicted Friday about whether Georgian or Russian forces had won control of the capital of the rebel province, South Ossetia. It was unclear late Friday whether ground combat had taken place between the two sides in the capital, Tskhinvali.

Georgia accused Russia of unleashing an air bombing campaign and claimed that hundreds of civilians had been killed; Russia denied those accusations.

Georgia is a close American ally whose shift toward the West and pursuit of NATO membership has angered Russia. The United States said Friday that it would send an envoy to the region to try to broker an end to the fighting in South Ossetia, and the European Union, NATO and Germany all called on both sides to stand down.

Secretary of State Condoleezza Rice issued a strongly worded statement Friday. “We call on Russia to cease attacks on Georgia by aircraft and missiles, respect Georgia’s territorial integrity, and withdraw its ground combat forces from Georgian soil,” Ms. Rice said in a statement released by the State Department.

The clashes raised the specter of a wider conflict in the Caucasus region, a key conduit for the flow of oil from the Caspian Sea to world markets and an area where violent conflict has flared for years along Russia’s borders, most recently in Chechnya.

Analysts said that Georgia 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. Russia also may be seeking to draw attention away from Abkhazia, where it has been under pressure to allow a settlement between pro-Russian and pro-Georgian factions, analysts said.

Richard Holbrooke, the former American ambassador to the United Nations, said that Russia’s aims in the escalating conflict 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.”

Georgian officials asserted that Russian warplanes had attacked Georgian forces and civilians in the capital, 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 Tblisi, the Georgian capital, a military base in Marneuli, and airports in the cities of Delisi and Kutaisi.

“We are under massive attack,” he said.

A spokesman for the Russian president, Dmitri A. Medvedev, declined to comment on Russia’s military actions.

Eduard Kokoity, the president of South Ossetia, told the Russian news agency Interfax that hundreds of civilians had been killed in fighting in the capital, though the claim was impossible to verify.

South Ossetia and Abkhazia gained de facto independence from Georgia in the late 1990s, 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 came to power and made national unification a centerpiece of his agenda.

Since then an uneasy truce has reigned, with fighting between South Ossetia and Georgian forces erupting sporadically.

Earlier this year, Russia announced that it was broadly expanding support for the separatist region.

The latest clashes between the Georgian military and separatist forces began last weekend. 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 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.

German Chancellor Angela Merkel issued a statement calling on both sides to use “the greatest prudence and restraint" and “to halt the use of force immediately."

Germany has taken a leading role in trying to ease tensions between Russia and Georgia over the other breakaway Georgian province of Abkhazia. Germany’s foreign minister, Frank-Walter Steinmeier, traveled to Georgia, Abkhazia and Russia last month with a peace proposal.

Georgia recalled its ambassador to Moscow Friday afternoon, the official Russian news agency RIA-Novosti reported.

The refugee crisis appeared to intensify Friday as relief groups said thousands of refugees, mostly women and children, were now streaming across the border into the North Caucasus city of Vladikavkaz in Russia in an attempt to escape the fierce fighting.

Early Friday, Russia’s Channel 1 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 already on the ground 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 that was moving into the area.

Russian state television showed what it said was a destroyed Georgian army tank in Tskhinvali, its turret smoldering. The station showed smoke rising over hills near the capital and women and children huddled in a corrugated metal room, apparently a bomb shelter.

Russian peacekeepers stationed in South Ossetia said that 12 peacekeeping troops were killed Friday and that 50 were wounded in Tskhinvali.

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 her relatives there. She declined to give her name.

Some civilians fled north into the Russian region of North Ossetia. A nurse at First Municipal Clinical Hospital in Vladikavkaz said that around a dozen people wounded from fighting in South Ossetia were being treated at hospitals in the city.

In Gori, a city outside South Ossetia and about 12 miles from Tskhinvali, residents said there had been sporadic bombing all day near the border with the breakaway region.

In the center of Gori, numerous vibrations from the impact of bombs could be felt on Friday evening, residents said. One Russian bomb fell in Gori near a textile factory and a cell phone tower, leaving a crater about 12 feet in diameter, they said.

At the United Nations on Friday, diplomats continued to wrangle over the text of a presidential statement after attempts to agree to compromise language collapsed around 2:30 a.m. Friday, 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, singled out Georgia and South Ossetia as needing to cease hostilities and return to the negotiating table.

But one phrase in particular 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 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 with new language 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 due to meet for consultations at 3 p.m. Friday, but diplomats said it was unclear they would be able to reach a compromise formula. 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.

At the Pentagon and the American military’s European Command headquarters responsible for activities in the region, senior officials continued to monitor the situation in Georgia as it unfolded on Friday.

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 Georgian authorities, the Defense Department had received no requests for assistance, the officials said.

Michael Schwirtz reported from Gori, Georgia, and Anne Barnard from Moscow. Reporting was contributed by Andrew E. Kramer and Ellen Barry from Moscow, Nicholas Kulish from Berlin, Neil MacFarquhar from the United Nations, and Thom Shanker from Washington. C. J. Chivers also contributed reporting.