"In the past week, Russia increased the number of troops by 2,000 to 7,000 staff," Georgian Interior Ministry spokesman Shota Utiashvili told a news conference.
"We fear Russia is preparing provocations in South Ossetia," he said.
There was no immediate response from Russian authorities.
Utiashvili said dozens of Russian armored vehicles had been positioned in the disputed Akhalgori region, the southeastern corner of South Ossetia which Georgia insists should be returned to Tbilisi's control under a French-brokered ceasefire deal.
Russia sent troops and tanks into Georgia in August to repel an offensive by the Georgian military to retake pro-Russian South Ossetia, which threw off Tbilisi's rule in 1991-92.
Russia's powerful counter-strike drove the Georgian army out of South Ossetia, and Moscow's troops then pushed further into Georgia, saying they needed to prevent Georgian attacks.
The West condemned Russia for a "disproportionate response" and Russian troops have since pulled back from buffer zones around South Ossetia and a second breakaway region, Abkhazia.
The Kremlin has recognized both rebel regions as independent states and said it would station 7,600 troops there to provide security.
A 225-strong European Union mission is monitoring the ceasefire, patrolling the former buffer zone around South Ossetia up to its de facto border.
Russia says the mission will not be allowed to operate inside South Ossetia.