Russia is sending a unit from the army's railway force to the breakaway province of Abkhazia, the country's defence ministry has said.
The 300 unarmed troops are needed to help carry out repairs on the network, said the head of Abkhazia's railways.
The move has been denounced by Georgia which says Russia is planning a military intervention in the province.
Tensions between the states have been high since April, when a UN report said Russia shot down a Georgian drone.
Russian authorities insisted the plane was shot down over Abkhazia by Abkhaz rebels.
Russian defence ministry spokesman Alexander Drobyshevsky said the deployment was part of the humanitarian aid to Abkhazia envisioned by former President Vladimir Putin.
ABKHAZIA'S BITTER WAR
The Abkhaz minority demanded independence from Georgia after the collapse of the USSR in 1991
Several thousand people were killed before Georgian forces were driven out in 1993
About 250,000 Georgians were displaced by the fighting
"Work to restore road and rail communications and infrastructure has been organised in which units and special equipment of the Russian Railway Troops (without weaponry) are taking part," he said.
But Georgia's Deputy Foreign Minister Grigol Vashadze called the deployment an "aggressive" step.
"We view this as yet another aggressive step by Russia aimed against the territorial integrity of Georgia," he said.
"They are strengthening their military infrastructure in order to start intervention in Georgia."
Last month, Moscow accused Georgia of preparing to invade Abkhazia.
Russia has kept a peacekeeping force in the province and South Ossetia under an agreement made following wars in the 1990s, when the regions broke away from Tbilisi and formed links with Moscow.
There are around 2,000 Russians posted in Abkhazia, and about 1,000 in South Ossetia.
Many in Abkhazia believe that Kosovo's announcement of independence from Serbia in February provides a precedent for it to be recognised as an individual state.
Although the province has its own flag and postage stamps, it is not internationally recognised.