Georgian forces have shelled towns in the breakaway province of South Ossetia, killing three people, separatist officials say.
A full mobilisation of forces has been declared, a spokesman for South Ossetia's breakaway government said.
Georgian authorities blamed the clash on separatist forces, saying they had attacked a Georgian checkpoint.
South Ossetia, along with Abkhazia, is one of two Georgian regions controlled by separatists close to Russia.
South Ossetian officials say Georgia shelled the region's main city, Tskhinvali, and nearby villages overnight - breaching a ceasefire agreed by the two sides.
They say three people died and 11 were wounded.
"A general mobilisation has been ordered," spokeswoman Irina Gagloyeva told AFP news agency.
Georgia, which is seeking to restore its authority in South Ossetia and Abkhazia, accuses Russia of aiming to absorb the regions.
Moscow has kept a peacekeeping force in both provinces under an agreement made following the wars of the 1990s, when they broke away from Tbilisi.
There are around 2,000 Russians posted in Abkhazia, and about 1,000 in South Ossetia.
Tensions between Russia and Georgia have flared up recently, despite Russia lifting economic sanctions against Georgia earlier this month.