GORI, Georgia (MSNBC) - Russia reportedly captured the central city of Gori and its armored vehicles rolled deep into western Georgia on Monday, seizing a military base and several towns and opening a second front of fighting. Georgian President Mikhail Saakashvili said the Russian forces had effectively cut his country in half.

"The Georgian army is retreating to defend the capital," the Georgian government said in a statement, adding that it was "urgently seeking international intervention to prevent the fall of Georgia."

The U.N. Security Council called an emergency session at Georgia's request for later Monday — the fifth meeting on the subject in as many days.

Fighting also raged Monday around Tskhinvali, the capital of the separatist province of South Ossetia. Swarms of Russian planes launched new raids across Georgia, sending screaming civilians running for cover.

The incursions came despite a top Russian general's claim earlier Monday that Russia had no plans to enter Georgian territory. If it has taken Gori, which sits on Georgia's only east-west highway and just 60 miles from the capital Tbilisi, Russia would have effectively cut the country in half.

Georgian National Security Council chief Alexander Lomaia said it was not immediately clear if Russian forces would try to advance on Tbilisi.

The State Department said it was evacuating more than 170 U.S. citizens from Georgia as the conflict grew.

A spokesman said two convoys carrying Americans along with an undetermined number of family members of American diplomats based in Georgia left Tbilisi en route to neighboring Armenia. More convoys were being prepared in case other Americans choose to leave Georgia.

The two-front battlefield was a major escalation in the conflict that blew up late Thursday when Georgia launched a military offensive to regain control of South Ossetia.

Even as Saakashvili signed a cease-fire pledge with European mediators on Monday, Russia flexed its military muscle and appeared determined to subdue the small U.S. ally that has been pressing for NATO membership.

On Monday afternoon, Russian troops invaded Georgia from the western separatist province of Abkhazia while most Georgian forces were in the central region around South Ossetia.

Russian armored personnel carriers moved into Senaki, a town 20 miles inland from Georgia's Black Sea port of Poti, Lomaia said.

Russian forces also moved into the Georgian town of Zugdidi and seized police stations, while their Abkhazian separatist allies took control of the nearby village of Kurga, Georgia's Interior Ministry spokesman Shota Utiashvili said.

Russia has so far fended off a wave of international calls to observe Georgia's pleas for a truce, saying it must first be assured of Georgia's retreat from South Ossetia.

The United States is campaigning to get Russia to halt its attack and American officials have accused Russia of using the fighting to try to overthrow the Georgian government. President Bush, who has encouraged Georgia's efforts to join NATO, said he spoke with Russian Prime Minister Vladimir Putin and Russian President Dmitri Medvedev.

"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 earlier Monday in an interview with NBC.

In turn, Putin criticized the United States for airlifting Georgian troops back home from Iraq on Sunday at Georgia's request.

"It's a pity that some of our partners instead of helping are in fact trying to get in the way," Putin said at a Cabinet meeting. "I mean among other things the United States airlifting Georgia's military contingent from Iraq effectively into the conflict zone."

NATO, for its part, said it would listen to Russia at a special meeting set for Tuesday at its headquarters in Brussels, Belgium. NATO envoys will also meet with Georgia's foreign minister.

The Associated Press and Reuters contributed to this report.