TSKHINVALI, Georgia: Russia accused Georgia of “an open act of aggression” on Friday after the separatist South Ossetia region said Georgian forces had killed two people in a mortar and grenade attack.

The Russian-backed separatist administration called up military reservists and put its security forces on alert in response to an overnight clash in which, they said, Georgian forces fired at their capital, Tskhinvali, and nearby villages.

Georgia’s Abkhazia and South Ossetia broke from central rule during wars in the 1990s after the collapse of the Soviet Union. Russia, which has peacekeepers in both unrecognised regions, has provided moral and financial support for the rebels.

Since the start of the year Abkhazia, the bigger of the two breakaway republics, has been the main source of friction but tensions this week shifted to South Ossetia, which lies 100-km north of the Georgian capital, Tbilisi.

“Moscow considers it unacceptable when Tbilisi ... is committing undisguised acts of aggressions against South Ossetia, the Russian foreign ministry said in a statement.

“The recent military incidents will lead to a sharp escalation in the armed confrontation in the conflict zone,” it said. “Any further delays in resuming the negotiations process could lead to the most tragic consequences.”

A reporter working for Reuters counted 12 explosions in the central square of the Ossetian capital, Tskhinvali, in the early hours of Friday morning. Officials with bodyguards fled towards official buildings before the electricity was cut.

After the blasts, intensive gunfire, including the sound of grenade launchers, could be heard for about 30 minutes.

The separatists said in addition to two servicemen who were killed, 11 people were wounded, most of them civilians. Georgia said it sustained no casualties. Tbilisi denied it had attacked, saying it had only responded to separatist fire.

Neither Tbilisi nor the separatists admitted violating a ceasefire, blaming the hostilities on one another. Russian Foreign Minister Sergei Lavrov demanded on Friday that Tbilisi sign a non-aggression pledge on South Ossetia.

Russian news agencies quoted the head of Russia’s peacekeeping troops in the region as saying extra soldiers could be deployed there if the stand-off worsened.

The exchange of fire was one of the most intensive in months in South Ossetia, a volatile patchwork of Georgian and separatist-controlled enclaves that is also a regular flashpoint between Tbilisi and Russia.

“They (the Georgians) were using large calibre weapons: mortars and grenade launchers,” Irina Gagloyeva, chair of the separatists’ committee for information, told Reuters. “Yesterday, we announced a general mobilisation,” she said.

Georgian President Mikheil Saakashvili, who wants his ex-Soviet Caucasus state to join Nato and the EU, sees the re-integration of Abkhazia and South Ossetia as a top priority. Moscow’s envoy to the Nato headquarters in Brussels accused the west of colluding with Tbilisi to play down the build-up.

“I think a lot of Euro-Atlantic structures are actually covering up the adventurism of Mr Saakashvili who has repeatedly said he wants to restore the territorial integrity by force,” Dmitry Rogozin told Vesti-24 television station.

The high tension caused “profound concern”, the Organisation for Security and Cooperation in Europe’s chairman-in-office, Finnish Foreign Minister Alexander Stubb, said on Friday.