Mikheil Saakashvili, Georgia's president, had earlier offered an immediate halt to heavy fighting, which had injured at least 21 people on Thursday, and said he had ordered his forces not to return fire if attacked.
Russian peacekeepers in the region reported that shelling had stopped for at least four hours after Saakashvili's televised speech.
However, just hours later the agency quoted Eduard Kokoity, the leader of the de facto government of South Ossetia, as saying his forces were confronting Georgians in the outskirts of Tskhinvali.
"Fierce fighting is under way," he said.
Ria-Novosti news agency quoted an official speaking for Ossetia's ministry of emergency situations as saying: "Tskhinvali is being shot at by mortar and heavy weapons from the Georgian villages of Nikozi and Ergneti and some houses are burning."
An AFP news agency reporter said that two columns of Georgian forces were seen approaching South Ossetia from near the Georgian town of Gori.
One column contained 40 trucks of armed interior ministry troops, some towing heavy artillery pieces, while the other column consisted of about 15 armoured personnel carriers.
The RIA-Novosti agency had said that Boris Chochiyev, the deputy prime minister of South Ossetia, and Temur Yakobashvili, the Georgian re-integration minister would meet at the Russian peacekeepers' base in Tskhinvali.
"Georgia's step is absolutely incomprehensible and shows that the Georgian leadership has zero credit of trust," Yuri Popov, the Russian envoy sent to the region to mediate the talks, said.
Georgia's president had said in his televised address earlier in the day: "Let's stop this spiral of violence... Let's resume negotiations."
|Kokoity said his forces were battling Georgian troops on the edge of Tskhinvali [AFP]
Saakashvili also reiterated a previous offer of "practically limitless" autonomy for South Ossetia. He also proposed that Russia could be the guarantor for any deal.
Martin McCauley, a London-based Russia analyst, told Al Jazeera: "You can argue that the president of South Ossetia, who wants independence from Georgia, is deliberately provoking Tbilisi and is trying to suck Russia in."
South Ossetia and another breakaway Georgian region, Abkhazia, receive extensive political and financial backing from Moscow.
Tbilisi has repeatedly accused Russian peacekeepers of supporting the separatists, while Moscow has accused Georgia of planning a full-scale invasion to re-establish control over the region.
"Russia is responsible for what is happening now in the conflict zone. Because it's not us who supply the arms for the separatists," Georgia's Yakobashvili said.
But Grigory Karasin, Russia's deputy foreign minister, on Thursday dismissed Georgian claims that Russia was responsible for the fighting as "cynical and unfair".
"The responsibility for the new outbreak of tensions in the region rests upon the Georgian side,'' he said. "We have to remind them again that it is impossible to resolve the South Ossetian problem with military methods."