Russian President Dmitry Medvedev on Saturday ordered a tightening of security in Georgia's breakaway South Ossetia region after a bomb blamed by Moscow on Georgian special forces killed eight Russian troops.

Medvedev ordered a "painstaking investigation" into Friday's car bomb blast at a Russian military base in Tskhinvali after the separatist regime's interior minister told AFP that three civilians had also died, taking the toll to 11.

The Russian leader ordered the defence ministry, in coordination with Georgia's Moscow-backed separatist administrations, to take "all necessary steps to prevent criminal acts against Russian peacekeepers and the civilian population," the Kremlin said in a statement.

Earlier a Russian prosecution spokesman, Vladimir Markin, said there was "every reason to believe the explosion in Tskhinvali was arranged by Georgian secret services," Russian news agencies reported.

The charge was denied by Georgia's interior ministry, which said the blast might have been intended to delay a pull-back of Russian troops from a "buffer zone" around South Ossetia, part of a European-brokered peace plan.

"Eight Russian peacekeepers died (along with) three civilians," South Ossetia's interior minister Mikhail Mindzayev said. Russian and local officials had earlier said seven Russian troops were killed with seven more injured.

Mindzayev said that Russian security forces had picked up three suspects -- the civilians -- along with a car which was brought under Russian supervision to a Russian military base in the rebel capital Tskhinvali, where it exploded, injuring a fourth official.

"How could we have done this? How could we possibly have known that the Ossetians were going to take this car and then bring it to the headquarters" of Russian forces in South Ossetia, Georgian interior ministry spokesman Shota Utiashvili told AFP.

"The headquarters of the Russian forces in South Ossetia is one of the most well-protected facilities in the region," added Utiashvili.

"We don't have access to Tskhinvali. We don't have access to the buffer zone. How could Georgia have been behind this?"

"This was a tactic to delay the withdrawal," Utiashvili said.

The head of the Russian military's joint staff for the South Ossetia conflict zone was among the Russian soldiers killed, Russian media reported Saturday.

The attack came just three days after more than 200 European observers deployed in various parts of Georgia at the start of a major mission to monitor the ceasefire and oversee the pull-back of Russian troops.

Tensions have remained high around Georgia's two rebel zones following an August war between Georgia and Russia over the region of South Ossetia.

Russian forces are due to withdraw from buffer zones around Georgia's rebel regions of South Ossetia and Abkhazia by October 10 under the peace deal, by which at least 200 European Union monitors have also been deployed in Georgia.

Since the end of August's hostilities three Georgian policemen have been killed in attacks blamed by Tbilisi on separatist snipers.

On September 23 Georgia said it had shot down a Russian spy plane that was surveying a key oil pipeline near South Ossetia.