WASHINGTON (Reuters) - The United States told Russia on Friday to withdraw its forces from U.S. ally Georgia and stop its air attacks on the tiny Caucasus state following fighting in the breakaway region of South Ossetia.

“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,” U.S. Secretary of State Condoleezza Rice said in a statement.

Rice issued her statement as Georgia, a former Soviet state that now wants to join NATO, said it would declare martial law and battled to get control of the rebel enclave, which was fortified by Russian forces.

Georgia said Russian fighter jets bombed container tankers and a shipbuilding plant in the port of Poti, prompting Washington’s sharpest rebuke of Russia since the crisis began.

“We deplore the Russian military action in Georgia, which is a violation of Georgian sovereignty and territorial integrity,” U.S. Ambassador Zalmay Khalilzad told reporters at a U.N. Security Council meeting in New York.

The State Department summoned the Charge d’Affaires at Russia’s Embassy in Washington, Aleksander Darchiyev, to see Rice’s deputy John Negroponte, who pressed Moscow to stop its military activities in Georgia.

“The deputy secretary said that we deplore today’s Russian attacks by strategic bombers and missiles, which are threatening civilian lives,” said State Department spokesman Robert Wood of Negroponte’s meeting with the Russian diplomat.

“These attacks mark a dangerous and disproportionate escalation of tension,” he added.

Both Rice and the White House urged an immediate cease-fire in South Ossetia, and U.S. officials said they would send an envoy to the region to help mediate.

As fighting raged in and around the South Ossetian capital of Tskhinvali, Georgian President Mikheil Saakashvili said Russia and Georgia were at war.

Rice said the United States was working with its European partners to launch international mediation, and “we urgently seek Russia’s support of these efforts.”

Diplomatic sources said the U.S. envoy would be Deputy Assistant Secretary of State for European and Eurasian Affairs, Matthew Bryza, who is expected to join a mission that includes the European Union and the Organization for Security and Cooperation in Europe.

Georgia is at the center of a battle for influence between the United States and Russia in the Caucasus.

President George W. Bush spoke to Russian Prime Minister Vladimir Putin about the crisis while both men were in Beijing for the Olympics. Bush has called Georgia a “beacon of democracy” in a volatile region.

NATO MEMBERSHIP

The Russian intervention came as something of a surprise to U.S. military officials who spent recent days monitoring the fighting in South Ossetia and Russia’s military buildup.

“The build-up of forces was more than expected and they moved earlier than we thought they would,” said a U.S. military official who spoke on condition of anonymity.

American military planners reviewed contingency plans for the possible evacuation of up to 3,000 U.S. citizens from Georgia, including about 130 defense personnel there to train Georgian military forces for duty in Iraq.

Washington has pushed hard for NATO membership for Georgia, despite European misgivings over the state’s stability. Russia strongly opposes Georgia’s NATO bid.

Robert Hunter of the RAND Corporation, a former U.S. ambassador to NATO, said “no one wants to fight for Georgia.”

He said the United States had pushed too hard to try to get NATO membership for Georgia, and now Russia was trying to demonstrate, in part, that U.S.-led NATO efforts had their limits and Moscow’s interests must be taken into account.

Republican presidential candidate John McCain said Russia should pull its forces out. “The consequences for Euro-Atlantic stability and security are grave,” he said.

Democrat Barack Obama condemned Russia’s role. “What is clear is that Russia has invaded Georgia’s sovereignty, has encroached on Georgia’s sovereignty, and it is very important for us to resolve this issue as quickly as possible.”