"Georgian armed forces ceased fire in the Tskhinvali region," the statement said, adding that President Mikheil Saakashvili had ordered the ceasefire and that his note had been sent to the Russian side.

According to Georgian sources, all "military units" are to be withdrawn from "the conflict area" and Georgia expressed its "readiness to immediately start negotiations with the Russian Federation on a ceasefire and termination of hostilities."

Russia confirmed receiving the note. According to media reports, however, Moscow criticized the continuation of combat operations in the conflict region.

"There are indications that exchanges of fire are continuing and the Georgian forces have not been fully withdrawn from the conflict zone," Interfax quoted Russian Foreign Ministry as saying.

Trouble in Abkhazia?

Over 4,000 Georgian troops on Sunday massed near its breakaway region of Abkhazia, the Interfax news agency quoted Abkhaz rebel leader Sergei Bagapsh as saying.

UN Secretary General Ban Ki-moon warned on Sunday that fighting in Georgia's Abkhazia could be "dangerously destabilizing" to the region following reports that the breakaway territory was mobilizing for a military operation.

The United Nations said the fighting has spread beyond South Ossetia, where the Russian army engaged in fighting with Georgian troops since Thursday.

The UN Security Council -- in which Russia is one of the five permanent members -- started a new fourth round of consultations on Sunday to update itself of the conflict in Georgia.

The previous meetings ended without any results.

Mounting tensions

Ban said in a statement that he was "profoundly concerned" over mounting tensions in Abkhazia's zone of conflict, including the bombing of the Upper Kodori Valley and the ongoing military build-up along the security zone.

"In the context of the announcement by the Abkhaz de facto authorities of a military operation in the Upper Kodori Valley which could be dangerously destabilizing, I am calling for the exercise of maximum restraint by all concerned as well as the guarantee of the safety and security of the unarmed United Nations military observers."

Ban urges "all parties to immediately end hostilities and to engage, without delay, in negotiations to achieve a peaceful settlement."

Abkhazia on Saturday asked the UN to pull out its 15 military observers from the Upper Korori Valley, which was complied. The unit was moved to Sukumi.

Both South Ossetia and Abkhazia have been pressing for independence from Georgia.

Disproportionate force

NATO Secretary General Jaap de Hoop Scheffer on Sunday deplored the "disproportionate use of force" and Russia's lack of respect for Georgia's territorial integrity.

Scheffer also urged an immediate ceasefire after Russian troops backed by tanks and fighter jets seized control of the breakaway province of South Ossetia.

"The Secretary General of NATO repeated today his call for an immediate ceasefire," a spokeswoman for the North Atlantic Treaty Organisation (NATO) said.

"He also expressed his concerns about the disproportionate use of force and lack of respect for the territorial integrity of Georgia" she added.

His comments came as Georgian President Mikheil Saakashvili called on NATO and the United Nations to "stop Russian aggression," in an interview Sunday with a German newspaper.

"Above all I think they have one moral duty: to speak with a united voice and stop Russian aggression," he told the daily Rhein Zeitung, when asked about the role NATO and the UN should play in Georgia's conflict with Russia.

But NATO is keen not to be drawn into the conflict.

"NATO does not have a mandate to play a direct role in the Caucasus," one Alliance official said.