President Bush held a press conference Monday denouncing Russia's escalation of violence in the former Soviet republic of Georgia. In a Rose Garden statement, he pressed Moscow to accept an immediate cease-fire and to pull back its troops from the conflict zone.
"Russia has invaded a sovereign neighboring state and threatens a democratic government elected by its people. Such an action is unacceptable in the 21st century," Bush said.
Georgian President Mikhail Saakashvili signed a cease-fire pledge Monday, and at Georgia's request, the U.N. Security Council in New York called an emergency session -- the fifth meeting on the fighting in as many days.
Russian armored vehicles charged into Georgian territory on two fronts today, seizing a military base and four cities, despite diplomatic efforts to halt the fighting and Georgia's president signing a cease-fire agreement.
The Russian offensive had troops poised just inside the rebel province of South Ossetia at the border of central Georgia, but in a surprise maneuver, Russia sent armored units across the border into western Georgia.
A top Russian general told ABC News that the army had no intention of invading Georgian territory.
The invading tanks spilled out of Abkhazia, a second province that has broken away from Georgia, to seize a Georgian military base and a couple of towns near the Black Sea.
Georgian officials and the Echo Moskvy radio station later reported that Russian troops had also surged across the South Ossetian border and taken the town of Gori in central Georgia, a move that could split Georgia in half.
An ABC News team left Gori just hours before the Russians arrived and reported the town was nearly empty except for Georgian soldiers, who were streaming out of town on armored personnel carriers and pickup trucks, yelling that the Russians were coming.
Saakashvili visited the town briefly with the French foreign minister before he was bundled into a car and driven away amid rumors of an imminent attack. The few civilians who remained ran from the streets in panic.
The two-pronged offensive came, despite calls by the European Union to halt the fighting and despite rising tensions between Russia and the United States.
The military showdown has triggered a war of words between Bush and Russian Prime Minister Vladimir Putin.
Bush, who was attending the Beijing Olympics, told NBC Sports that the Soviet offensive was unacceptable.
"I've expressed my grave concern about the disproportionate response of Russia, and that we strongly condemn the bombing outside of South Ossetia," Bush said. "I was very firm with Vladimir Putin." Bush said he made the same point later in a phone call to Russian President Dmitri Medvedev.
Vice President Dick Cheney was more blunt, telling Georgia's president that "Russian aggression must not go unanswered, and that its continuation would have serious consequences for its relations with the United States."
Asked to explain Cheney's phrase "must not go unanswered," White House spokesman Gordon Johndroe said, "It means it must not stand." White House officials refused to indicate what recourse the United States might have if the attacks continue.
Putin fired back at the U.S. today, indicating he was particularly upset at the American military for flying 2,000 Georgian troops from Iraq, where they were helping out their U.S. allies, back to Georgia.
"It is a shame that some of our partners are not helping us but, essentially, are hindering us," Putin said. "I mean ... the transfer by the United States of a Georgian contingent in Iraq with military transport planes practically to the conflict zone."
Putin's emotional speech said the U.S. was wrong to blame Russia for the outbreak of war.
"The very scale of this cynicism is astonishing -- the attempt to turn white into black, black into white and to adeptly portray victims of aggression as aggressors and place the responsibility for the consequences of the aggression on the victims," he said.
The U.S. appears to have little leverage over Russia, however, in part because the Bush administration needs Russian cooperation in its negotiations with Iran over its nuclear program.
After Saakashvili signed a cease-fire agreement , he later held a news conference to accuse Russia of trying to overthrow his government.
"Russia made clear the goal of this operation is the regime change in Georgia, change of democratically elected government," he claimed. "It is quite clear what is happening. We are in the process of an invasion, occupation and annihilation of an independent democratic country."
He called the Russian offensive a "pre-planned, cold blooded, meditated murder of a small country."
Saakashvili compared Russia's push to its invasion in 1968 of Czechoslovakia and Afghanistan in the 1990s.
"As we speak, there are people being shot, killed," he said. He later added, "What else can happen for the world to wake up and see what's at stake?"
Russia and Georgia accused each other today of carrying out campaigns of ethnic cleansing of the breakaway provinces.
Despite facing an overwhelming military force, Saakashvili, when asked what he would do if Russian forces moved toward Tblisi, said, "We have no plan of surrender."
EU officials hope to convince Russia to also sign the cease-fire deal. The foreign ministers from the G-7, representing the world's seven largest economic powers, called on Russia to accept the cease-fire with Georgia, following a conference of all the ministers, including U.S. Secretary of State Condoleezza Rice.
Besides backing its Georgian allies, the U.S. is closely watching the conflict because of Georgia's status as a major conduit of oil to Black Sea ports. Any disruption to Georgia's main pipeline could send world oil prices soaring again, and, in turn, kick up the price of gas at U.S. pumps. So far, the pipeline has been untouched.
The outbreak of fighting has reportedly claimed the lives of 2,000 civilians caught in the crossfire, and thousands more are homeless, with many fleeing the area. The capital of South Ossetia has been devastated by shelling.
Russia has deployed ships off the Georgian coast and claims to have sunk a Georgia missile ship, while swarms of Russian planes bombed sites near the Georgian capital of Tbilisi for the fourth straight day.
Russia claimed that Georgian artillery again shelled the South Ossetian capital Tskhinvali, and claimed that two more of its planes have been shot down, bringing the total of lost Russian planes to four.
Russia insists that it was withdrawing from some of its positions in Georgia, but people remain fearful and expect more attacks.
The shooting began when Georgian troops moved last week to reassert control of its breakaway province, South Ossetia. Georgian troops briefly occupied the South Ossetian capital before a powerful Russian force chased them back into Georgia.
The Associated Press and Reuters contributed to this report.