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Sergei Grits, Associated Press
Bodyguards escort President Mikhail Saakashvili, third from left and wearing a protective vest, to shelter under the threat of Russian air attack in Gori. Some say the Kremlin is trying to oust Saakashvili.

WASHINGTON — Russian troops stepped up their advance into Georgian territory on Monday, attempting to turn back the clock to the days when Moscow held uncontested sway over what it considers its "near abroad" and arousing increasing alarm among Western leaders.

Even as they prepared to convene an emergency meeting of NATO today and President Bush denounced the Russian actions in the strongest terms to date, the United States and its European allies faced tough choices over how to push back.

Russian tanks roared deep into Georgia, launching a new western front in the conflict, and Russian planes staged air raids that sent people screaming and fleeing for cover. Russian troops briefly seized a Georgian military base and took up positions close to the Georgian city of Gori on Monday, raising Georgian fears of a full-scale invasion or an attempt to oust the country's pro-Western president, Mikhail Saakashvili.

Saakashvili told CNN late Monday that Russian forces were cleansing Abkhazia of ethnic Georgians. "I directly accuse Russia of ethnic cleansing," he said. At the U.N. on Friday, each side accused the other of ethnic cleansing.

Bush, little more than an hour after returning to Washington from the Olympic Games in Beijing, bluntly warned Russia that its military operations were damaging its reputation and were "unacceptable in the 21st century."

"Russia's actions this week have raised serious questions about its intent in Georgia and the region," he said. "These actions have substantially damaged Russia's standing in the world, and these actions jeopardize relations with the United States and Europe."

Administration officials said military options were almost certainly off the table, but the United States did airlift Georgian troops stationed in Iraq back home, answering a plea from the Georgian government and prompting a sharp response from Russia. Washington could also press to ostracize Moscow on the international stage, perhaps by kicking it out of the Group of 8 industrialized nations.

Yet there was no immediate indication that Western powers could exercise much leverage over Russia if it chooses to ignore their warnings. The country is enjoying windfall profits from oil exports and seems determined to reassert influence over Georgia and Ukraine, while sending a clear signal to former satellite states that they should be wary of an overly cozy political and military alliance with the United States, analysts say.

"If the United States and Europe don't stop Russia, I think this is the end of what we thought of as the post-Soviet era," said Sarah Mendelson, a scholar at the Center for Strategic and International Studies.

George Friedman, chief executive of Stratfor, a geopolitical risk analysis company, said: "The Russians feel they have been treated like dirt by the world for the last 20 years. Now, they're back."

Many experts in foreign policy say that one reason Russia responded so forcefully to Georgia's attempt to take back South Ossetia is that the United States and Europe had been asserting themselves in Russia's backyard, alienating Moscow by supporting Kosovo's bid for independence.

These experts say that the Bush administration's efforts to promote democracy, including backing Saakashvili as a beacon of democracy on Russia's borders, may have emboldened the Georgian president to take provocative actions that brought a fierce Russian response.

Beyond that, Russia has also been angry about American plans to put a missile defense system in Poland, and by American moves to encourage Georgia and Ukraine to join NATO.

"The combination is that the overall means with which we've dealt with the Russians over the last two years have painted them into a corner so that it's difficult for them not to see us as hostile," said Michael Greig, conflict management specialist at the University of North Texas.

Few foreign policy experts predict that Russia will ever recapture its days of communist glory, global intimidation and military might; the world has changed and growing global powers like China and India will make a return to the Cold War impossible.

But there is a growing belief in European capitals and in Washington that the return of Russia could mean a distinct redrawing of the Eurasia map, with Europe and the United States giving up on attempts to integrate former Soviet republics in the Caucasus, into the Western orbit, while battling with Russia to keep Eastern European countries like Poland and the Baltic states.

And Russia's resurgence could mean an end to already-dwindling American and European hopes of bringing Russia along eventually as an ally of the West. At best, Russia would never be trusted; at worst, it would be seen as an adversary.

Even for an emboldened Moscow, the Russian foray into Georgia carries substantial risks: not just global isolation from the Western democracies, but also anger from neighboring states of the former Soviet Union and Warsaw Pact, the prospect of perpetual military quagmires around its borders and nationalist reprisals like those that resulted from its crackdown in Chechnya.

A crowd of more than 1,000 people demonstrated in the Latvian capital, Riga, on Monday, while hundreds gathered in Tallinn, Estonia, and Vilnius, Lithuania, to press the West to adopt a tough stance toward Moscow. Leaders in Poland and the Czech Republic echoed that call.

Even as American and European leaders were demanding, begging and pleading on Monday with Russia to halt its advance into Georgia — foreign ministers from the world's richest countries held an emergency conference call and notably excluded Russia's foreign minister by limiting the group to the G-7, instead of the G-8 — diplomats were going through what one Bush administration official described as "not exactly the greatest hand of cards to have to play."

At the United Nations, the Russians were dismissive of a draft resolution to end the fighting, which began to circulate among Security Council members. The Russians said they were disappointed that they had not been consulted on the agreement as it was being drawn up and noted that there was no mention of "Georgian aggression."

Secretary of State Condoleezza Rice sent a midlevel State Department official, Matt Bryza, to the region to back up mediation efforts that are being led by the foreign minister of France, Bernard Kouchner. Georgian officials urged their European counterparts to take more punitive steps, like ending plans to pursue a new strategic partnership with Moscow, and questioning the 2014 Winter Olympic Games in Sochi, Russia.

The games in Sochi are a personal project for Russia's prime minister, Vladimir V. Putin, who favors Sochi as a summer and winter retreat, and skis in nearby mountains, close to the border with disputed Abkhazia.

But Democratic critics of the Bush administration criticized the administration's moves so far as weak. Richard C. Holbrooke, the former U.S. ambassador to the United Nations, noted that President Nicolas Sarkozy of France was leading the efforts to mediate.

Rice, Holbrooke said, should be on a plane to Moscow, particularly given the administration's close ties to Georgia, and its encouragement of that country's efforts to join NATO.

But the problem has become the response: Russia has now pushed back hard, and the United States, bogged down in Iraq and Afghanistan and fretting about Iran, is unlikely to take on Russia over the matter of Georgia. Russia has shown that it wants to rule its own backyard, said Friedman of Stratfor.

"All this basically means that Russia emerges as a great power," Friedman said. "Not a global power like it used to be, but a power that has to be taken very seriously."

Russia's Deputy Foreign Minister Grigory Karasin said more than 2,000 people have been killed in South Ossetia since Friday, most of them Ossetians with Russian passports.

Contributing: Marc Santora, Steven Lee Myers and C.J. Chivers, New York Times News Service; Christopher Torchia and David Nowak, Associated Press