Georgian forces and South Ossetian separatists have been exchanging heavy fire just hours after agreeing to a ceasefire and Russian-mediated talks.
Georgian forces and South Ossetian separatists have been exchanging heavy fire just hours after agreeing to a ceasefire and Russian-mediated talks.
Fighting broke out after a brief lull in a day of heavy fighting in which 12 people were reportedly killed.
The BBC's Matthew Collin in Tbilisi says the separatist stronghold of Tskhinvali appears to be under siege, with a series of huge explosions.
Russian media reported that hundreds of fighters from Russia and Abkhazia were heading to aid the separatist troops.
Georgia says an operation is under way to restore constitutional order.
Russia, which has close ties with South Ossetia, has claimed Georgia is ready to overrun the breakaway province.
But the Russian-backed separatists have appealed to Moscow and the world to protect them "extermination".
Moscow-based Interfax news agency quoted the head of Russia's province of North Ossetia, Taimuraz Mamsurov, as saying: "Hundreds of volunteers from North Ossetia are on their way to South Ossetia. We cannot stop them or prevent them from going."
Russia has called on the international community to jointly work to "to avert massive bloodshed and new victims".
It also quoted Sergei Bagapsh, the head of Georgia's other breakaway province of Abkhazia, as saying: "About 1,000 Abkhaz volunteers are leaving for South Ossetia."
At Moscow's request, members of the UN Security Council are expected to hold a rare emergency session shortly to discuss a response to the escalating violence.
Hundreds of fighters from Russia and Georgia's other breakaway region of Abkhazia were heading to aid the separatist troops, Russian news agencies reported.
Meanwhile, the head of Georgian peacekeepers in South Ossetia said they were going to "restore constitutional order" to the region.
Meanwhile, the head of Georgian peacekeepers in South Ossetia said they were going to "restore constitutional order" to the region.
"We had demanded that [the separatists] sit at the negotiating table but all of this was met with a backwards reaction and there was constant shooting," said Brig Gen Mamuka Kurashvili.
The Georgian minister for integration, Temur Yakobashvili, said Georgia's aim was not to take over the territory but to finish "a criminal regime".
'Perfidious step'
'Perfidious step'
Earlier, South Ossetian rebel leader Eduard Kokoity told Interfax that Georgia had launched an all-out attack on the town of Tskhinvali, in what he called "a perfidious and base step".
South Ossetian rebel leader Eduard Kokoity told Russia's Interfax news agency that Georgia had launched an all-out attack on the town of Tskhinvali, in what he called "a perfidious and base step".
Up to 10 Georgian soldiers died in Thursday's clashes, the Georgian interior ministry told AFP news agency.
Up to 10 Georgian soldiers died in Thursday's clashes, the Georgian interior ministry told AFP news agency.
Separatist authorities claimed on their website two civilians had been killed after coming under Georgian attack.
Separatist authorities claimed on their website two civilians had been killed after coming under Georgian attack.
Days of fierce conflict have raised fears of a new war in the volatile Caucasus.
Days of fierce conflict have raised fears of a new war in the volatile Caucasus.
In a televised address, Georgian President Mikhail Saakashvili earlier called for the bloodshed to end, but also warned the separatists not to try Tbilisi's patience.
In a televised address, Georgian President Mikhail Saakashvili earlier called for the bloodshed to end, but also warned the separatists not to try Tbilisi's patience.
A truce was later announced, with emergency talks set for Friday, but by nightfall, both sides were trading heavy fire.
A truce was later announced, with emergency talks set for Friday, but by nightfall, both sides were trading heavy fire.
Georgia accuses Russia of arming the South Ossetian authorities - who have been trying to break away since the civil war in the 1990s. Moscow denies the claim.
Georgia accuses Russia of arming the South Ossetian authorities - who have been trying to break away since the civil war in the 1990s. Moscow denies the claim.
President Saakashvili has vowed to restore Tbilisi's control over South Ossetia and another breakaway province, Abkhazia.
President Saakashvili has vowed to restore Tbilisi's control over South Ossetia and Abkhazia.
The Ossetians have traditionally had good relations with Russia, and North Ossetia is part of the Russian Federation.
The Ossetians have traditionally had good relations with Russia, and North Ossetia is part of the Russian Federation.
Russia is hostile to Georgia's ambition to join Nato and has accused Georgia of building up its forces around the breakaway regions, where Russian peacekeeping troops are deployed.
Russia is hostile to Georgia's ambition to join Nato and has accused Georgia of building up its forces around the breakaway regions, where Russian peacekeeping troops are deployed.
Are you in South Ossetia or elsewhere in Georgia? Have you been caught up in events? Send us your comments using the form below:
Are you in South Ossetia or elsewhere in Georgia? Have you been caught up in events? Send us your comments using the form below:
