Russian and Georgian armed forces were locked in combat on Friday over control of the Caucasus region South Ossetia, with hundreds of civilians reported killed or injured, dpa reported.
The escalating crisis prompted calls for restraint from international governments and bodies including the United States, European Union, NATO and the Organization for Security and Cooperation in Europe (OSCE).
Eduard Kokoity, South Ossetia's leader, said more than 1,400 civilians had died as a result of the combat beginning in sporadic firefights on August 1, and shifting to full-blown warfare early on Thursday.
Georgian fire killed at least 10 Russian peacekeepers stationed in the breakaway Georgian province in severe fighting centered in the South Ossetian city Tskhinvali, Russian army spokesmen said.
Georgian President Mikheil Saakashvili in a nationwide television statement said his country's troops controlled a "large part of South Ossetia" and have "liberated" Tskhinvali.
Russian army sources contradicted the Georgian president, saying Russian artillery and tank fire had halted a Georgian offensive into the region begun Thursday, inflicted substantial casualties on Georgian forces, and were assisting a South Ossetian counterattack.
Artillery, mortars, tanks, ground attack aircraft, helicopters, and salvo rocket launchers were among the weapons reported used by one or both of the warring sides in house-to-house fighting in the city, and in regions around it.
Russian artillery deployed on the north side of Tskhinvali fired directly on Georgian artillery in the mid-afternoon, forcing a Georgian retreat from the city centre, according to Russian army spokesmen.
Georgian television early evening on Friday aired images seemingly showing Tskhinvali's centre fully under Georgian control.
Georgian media also reported a pair of Russian aircraft shot down during Friday - a claim flatly denied by the Kremlin.
Russian air force combat operations over Georgia appeared however to be an established fact, with a strike by Su-25 ground attack aircraft against a Georgian military base near the town of Gori confirmed by Georgian officials, without giving information of the effects of the bombing.
Tanks, armoured personnel carriers, self-propelled artillery and support elements of Russia's 58th army began entering South Ossetia by mid-morning. Russian diplomats described the de facto intervention as "support to Russian peacekeepers", and "protection to the local population."
News reports from Russia and Georgia both told of massive damage to offices and homes throughout Tskhinvali. Eyewitnesses described entire city blocks as flattened by shelling.
Georgian television showed images of soldiers wounded in the fighting, but gave no casualty numbers.
Russian President Dmitry Medvedev speaking earlier in the day said Russia had no choice but to take action to protect its citizens.
"We will not let the deaths of our citizens go unpunished, the guilty will incur the punishment they deserve," Medvedev said in his first statement since heavy fighting erupted, claiming six casualties in weekend clashes.
On the Georgian side, Saakashvili's national televised address called for full mobilization: "Hundreds of thousands of Georgians should stand together to save Georgia."
Television images Friday showed long Georgian military convoys heading towards South Ossetia and also Abkhazia, a second renegade Georgian province supported by Moscow.
International efforts to halt the fighting, possibly the most severe seen in the Caucasus region in a generation, came to little.
US Secretary of State Condoleezza Rice according to Saakashvili called "several times" throughout the day, and the US State department announced it was sending an envoy to Georgia.
"We support Georgia's territorial integrity and we call for an immediate ceasefire," State Department spokesperson Amanda Harper told Deutsche Presse-Agentur dpa.
The US was working on "mediation efforts" to help secure an end to the hostilities, Harper said.
The EU called for an immediate ceasefire and a return to peace talks.
The EU "calls all parties to cease hostilities immediately and to resume talks without delay in order to permit a political solution to the crisis while respecting the sovereignty and territorial integrity of Georgia," a statement on behalf of the EU by the French government, which currently holds the bloc's presidency, said.
"The EU is working with other actors to achieve a cease-fire in order to prevent an extension of the conflict," the statement said.
Ahead of a special meeting in Vienna of the 56-nation OSCE, current chair Finland warned that "war would have a devastating impact for the entire region."
Georgia's ambassador to the OSCE Victor Dolidze told the OSCE assembly that Russia had bombed Georgian territory since Friday morning, a diplomat said.
Russia's involvement in the conflict was "clear, open, military aggression from one OSCE country to another, its neighbour," Dolidze said.
German Chancellor Angela Merkel also expressed concern Friday about the situation, callingon the parties to the conflict to exercise calm and end immediately the violence, a German government spokesman said.
In Brussels, NATO Secretary General Jaap de Hoop Scheffer said that NATO is "closely following" the situation, a statement said.
De Hoop Scheffer "calls on all sides for an immediate end of the armed clashes and direct talks between the parties," it added.
Council of Europe Secretary General Terry Davis meanwhile said in a statement: "The human toll of the escalation of the conflict in Georgia continues to rise and the country is now on the edge of a full-scale war which would have devastating consequences for the people in the region."
The United Nations Security Council scheduled another emergency meeting Friday afternoon in New York, after failing Thursday night to agree on a response to the escalation of violence.
Pro-Western Saakashvili, who with US backing is bidding for Georgia to join NATO, has made re-asserting control over both the rebellious regions a top priority of his presidency.
South Ossetia had become a lawless region ruled by bandits and dangerous to regional stability, and Georgia's right to assert state control over the province was guaranteed under the Georgian constitution, Saakashvili argued.
Moscow, in turn, has been angered by the former Soviet country's rapprochement with the alliance and moved to strengthen ties with the rebel government. The Kremlin's support to South Ossetia has long been seen in Tbilisi as a first Russian step towards de facto annexation of the Georgian territory.
South Ossetia is inhabited primarily by Ossetians, an Iranic ethnicity traditionally opposed to both Georgian and Russian efforts to assert control of the region.
Over 2,500 Russian peacekeeping troops have been stationed in the province since a 1994 ceasefire ending fighting between the Georgian army and Ossetian separatists.