Russian forces are advancing towards Tbilisi, according to the Georgian government, despite a fragile ceasefire being agreed by the two sides.
The reports on Sunday come hours after Russian troops were seen beginning to withdraw from Kaspi and Igoeti, towns close to the Georgian capital.
Fighters from the breakaway region of Abkhazia, who are backed by Moscow, have also taken control of 13 Georgian villages and a hydropower plant, according to the Tbilisi government.
Jonah Hull, Al Jazeera's correspondent in Tblisi, said: "This is indeed disturbing news.
"The word from Moscow is that the Russians have come under attack by the Georgians. But the Georgians are saying this is a pretext for Russia ... to move a column of armour toward Tblisi.
"This will destabilise the situation enormously, if it is true."
Georgia has also accused Russia of blowing up a key railway bridge in the Kaspi region and "paralysing the Georgian rail network", a claim backed by eyewitnesses.
But Colonel-General Anatoly Nogovitsyn, the deputy chief of Russia's General Staff, said: "We are now in peacetime. Why should we be blowing up bridges when our job is to restore?"
The events come after Dmitry Medvedev, Russia's president, signed a six-point ceasefire deal earlier on Saturday pledging to end the conflict with Georgia.
The agreement calls for an end to all military action, free access to humanitarian aid, Georgian troops to return to their places of permanent deployment and Russian forces to go back to the positions they held before fighting began in South Ossetia.
It also calls for a demilitarised border zone.
Sergei Lavrov, the Russian foreign minister, confirmed that Medvedev signed and ordered the implementation of the ceasefire deal, but said Russian troops would not withdraw until Moscow was satisfied that security measures it forces are allowed to take under the agreement are effective.
"As these additional security measures are taken, the units of the Russian armed forces that were sent into the zone of the South Ossetian conflict ... will be withdrawn," he said.
"We are constantly encountering problems from the Georgian side, and everything will depend on how effectively and quickly these problems are resolved."
Russian troops were also reported to be still outside the strategically important town of Gori, 60km northwest of Tbilisi on Saturday afternoon. The town of 55,000 people has been deserted by its residents.
Georgian men aged in their 40s and 50s have been forced to clear debris from the streets of separatist South Ossetia's bombed-out capital, where Ossetians have accused them of targeting civilians in the August 7 attack by Tbilisi on the region.
"They are cleaning up after themselves," Mikhail Mindzayev, South Ossetia's interior minister, told the AP news agency.
George Bush, the US president, has stressed the two regions at the heart of the bloody conflict are "a part of Georgia," and "will remain so".
Al Jazeera's Neave Barker, reporting from Moscow, said that the ceasefire pact was being hailed as a success for Russia.
"Quite clearly, what Saakashvili has agreed to is a reduction in any influence, on the Georgian side, in these two areas [South Ossetia and Abkhazia]."
Condoleezza Rice, the US secretary of state, will travel to Brussels next week to meet Nato foreign ministers and EU officials in an effort to bolster support for Georgia.
Angela Merkel, the German chancellor, had earlier secured the agreement of Medvedev to the EU-brokered deal, but was told Moscow would guarantee the will of the people in the pro-Russian separatist regions.
|Saakashvili, right, has held hours of talks with Rice [AFP]
Russian troops entered Georgia following Tibilisi's massive barrage to try to take control of South Ossetia on August 7.
The province had achieved de facto independence from Georgia in the 1990s during the breakup of the Soviet Union.
Even if Russian forces do withdraw from the rest of Georgia, Moscow appears likely to maintain strong control over the provinces.
Lavrov said on Thursday that Georgia can "forget about" South Ossetia and Abkhazia, which broke from Georgian government control during wars in the early 1990s.
Moscow, which firmly backs South Ossetia and Abkhazia, has issued Russian passports to most people in the two territories.