TBILISI--Georgia withdrew from the separatist region of South Ossetia on Sunday after new clashes with Russian forces as Moscow amassed its military firepower to overwhelm its neighbor.

"We have left practically all of South Ossetia as an expression of goodwill and our willingness to stop military confrontation," Georgian National Security Council Secretary Alexander Lomaia told AFP.

Officials in South Ossetia, the separatist region at the heart of the fighting, said artillery fire was exchanged overnight and Georgia claimed Russian jets had bombed a military airfield near the Georgian capital Tbilisi.

Georgia also said Russia had brought 10,000 extra troops into South Ossetia and was assembling armored vehicles close to the border. Reports said Russia was imposing a naval blockade after moving warships into range.

Lomaia said Georgia had asked the United States to act as a mediator with Russia in the conflict, which broke out in the middle of last week and has left 2,000 dead according to Russian figures.

"We have asked United States Secretary of State Condoleezza Rice to mediate with the Russians, to transmit them our message," said Lomaia, after announcing the withdrawal.

US President George W. Bush has led a chorus of international calls to end the hostilities which observers fear might spread to other parts of the volatile Caucasus region.

Russia backs the separatist government in South Ossetia and sent in tanks and troops on Friday in response to pro-Western Georgia's military offensive to take back the province which broke away in the early 1990s after a separatist war.

In a new move demonstrating Moscow's giant military advantage, Russian warships were reported by Interfax news agency to be setting up a sea blockade to prevent arms and other military supplies from reaching Georgia.

"This is definitely necessary for preventing arms shipments to Georgia by sea," Interfax quoted a naval source as saying. "A sea blockade of Georgia will also help avert an escalation of military activity in Abkhazia."

Overnight, Russian and Georgian forces exchanged artillery fire in the South Ossetian capital Tskhinvali, the South Ossetian authorities said in a statement. A South Ossetian spokeswoman said the overnight shelling killed 20 and wounded 150 people.

Russia also continued bombing raids on Georgian territory outside South Ossetia, hitting a runway of a military airfield near Tbilisi international airport early Sunday, according to Lomaia.

On Saturday, Russian aircraft had staged raids on the Georgian port of Poti and the city of Gori, where inhabitants said scores of people were killed.

Lomaia said Russia had concentrated a large number of armored vehicles near the border with Georgia at a crossing not linked to South Ossetia.

Georgia's army of less than 25,000 men is confronting a Russian force which can count on more than one million troops -- and experts say the conflict cannot last.

Diplomatic efforts to end the crisis were set to intensify, with a joint European-US mission heading for Georgia to try to help broker a ceasefire with Russia.

But a meeting of the UN Security Council on Saturday failed to agree on a call for an immediate ceasefire.

France, which holds the EU presidency, announced that it would organize a meeting of European foreign ministers early next week and an emergency EU summit could be held.

The European Union "strongly states its commitment to the sovereignty and the territorial integrity of Georgia and its internationally recognized borders and urges Russia to respect them," said a statement released by France.

The EU "underscores that the military actions (against Georgia) could affect EU-Russian relations," it added.

French Foreign Minister Bernard Kouchner will visit the region on Sunday to present settlement proposals, France said.

US President George W. Bush cut into his engagements during a visit to Beijing to call for an end to Russian bombing.

"We have urged an immediate halt to the violence and a stand-down by all troops," Bush told reporters. "We call for an end to the Russian bombings."

As the fighting continued, the leaders of Russia and Georgia stepped up their war of words.

"What they are doing is nothing to do with conflict, it is about annihilation of a democracy on their borders," Georgian President Mikheil Saakashvili said in an interview with the BBC on Saturday.

Saakashvili declared a "state of war" in his country on Saturday but also offered a ceasefire to Russia.

Russia's President Dmitry Medvedev said his country had launched its operation to "force the Georgian side into peace." He later said he would order an investigation into accusations by South Ossetian refugees of Georgian acts of genocide.

The conflict has also spread to Abkhazia, another breakaway region of Georgia, where the separatist government said its forces had launched attacks on Georgian troops. Georgia accused Russia of staging the attacks in the Kodori Gorge region, the only part of Abkhazia controlled by Georgia.

The conflict with Russia has claimed 150 Georgian lives, Georgian Foreign Minister Eka Tkeshelashvili said Saturday.

Russian officials have said at least 2,000 people have been killed in South Ossetia.

South Ossetia broke from Georgia in the early 1990s. It has a population of 70,000, many of whom have been granted Russia passports.