Russia has about 2,000 peacekeepers in Abkhazia
Nato's secretary general has demanded that Russia withdraw troops it sent to the disputed breakaway Georgian province of Abkhazia last week.
Jaap de Hoop Scheffer accused Russia of breaching Georgia's sovereignty by sending in military railway personnel.
Mr de Hoop Scheffer said the Russian move was "contributing to instability in what is already a volatile area".
Abkhazia broke away from Georgia in a war in the early 1990s after the Soviet Union collapsed in 1991.
Russia announced on Saturday that it was sending 300 members of an unarmed unit from the army's railway force to the province to help carry out repairs on the network, labelling the move a humanitarian effort.
Georgia responded that Russia is planning a military intervention in the province.
"These forces should be withdrawn, and both Russia and Georgia should engage quickly in a high-level and open dialogue to de-escalate tensions," Mr de Hoop Scheffer said.
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
Georgia, which hopes to join Nato, has accused Russia of propping up separatists in the region with a peacekeeping force.
Tensions have been high since Moscow announced in April that it was establishing formal ties with the separatists.
Adding to those tensions is the release of a UN report that said Russia shot down a Georgian drone.
Russian authorities insisted the plane was shot down over Abkhazia by Abkhaz rebels.
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 Georgia 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.