* Danger of skirmishes
* No Russian pullback from rebel regions
TBILISI/TSKHINVALI, Georgia, Aug 4 (Reuters) - A dangerous security vacuum in Georgia's rebel regions and an unfulfilled ceasefire pact threaten renewed hostilities a year after the Caucasus country's five-day war with Russia.
With the withdrawal of military observers from South Ossetia and Abkhazia, little has been done to confront the danger of skirmishes boiling over into full-blown hostilities, analysts warn. Unarmed European Union monitors are denied access.
Friday marks the anniversary of Georgia's assault on pro-Moscow South Ossetia, and Russia's crushing counter-strike.
A year on, Georgian police hold positions behind sandbags a few hundred metres from the South Ossetian capital Tskhinvali, while Russian FSB security service officers in camouflage uniform control the borders of the rebel territory 50 km (30 miles) from Tbilisi at their nearest point.
"We came for a long time and we're ready to defend this republic," said Pavel Bozhov, a border patrol officer of the FSB, which is successor to the KGB.
Hansjoerg Haber, heading 240 EU monitors deployed after the war but patrolling only as far as the boundary, said they were succeeding in "refreezing the conflict".
"But if we don't introduce a dynamic element, like confidence building, it could at some stage re-erupt."
The lead-up to the anniversary has seen a spike in tensions.
South Ossetia and Georgia have traded accusations of mortar rounds and rocket-propelled grenades fired over the boundary.
Russia warned on Saturday it would use "all available force" to defend South Ossetia, a statement Georgia said demonstrated Moscow's "dangerous designs".
"The biggest danger is probably less some pre-planned military campaign or intervention, but a situation which starts as a local conflagration and goes out of control," said Lawrence Sheets of the International Crisis Group thinktank.
Georgia launched an assault on South Ossetia on Aug. 7 after days of skirmishes and months of tension between Moscow and Tbilisi over South Ossetia and the rebel Black Sea region of Abkhazia, which both broke away in wars in the early 1990s with the collapse of the Soviet Union.
Russia responded with a devastating counter-strike that routed the Georgian military. Russian forces pushed into Georgia proper, shaking confidence in oil and gas routes running West through the former Soviet republic. An EU-brokered ceasefire called for forces to withdraw to pre-war positions.
But Russia has instead strengthened its military grip on landlocked South Ossetia and sub-tropical Abkhazia, which Moscow recognised as independent states. Russia says it has more than 7,000 troops based in the territories.
Monitors from the OSCE and United Nations have pulled out in a row between the West and Russia over sovereignty. South Ossetia and Abkhazia have denied EU monitors access.
"There are no active hostilities, but the crux of the document was aimed at reduction of troop levels and restoring the status quo, and that hasn't happened," said Sheets.
The stretch of boundary skirting Tskhinvali is a danger point, as is Georgian-populated Akhalgori in South Ossetia that was under Georgian control until the war. The mainly Georgian Gali region of Abkhazia is also a source of tension.
Tens of thousands of Georgians and South Ossetians were displaced by the fighting, and 25,000 Georgians remain unable to return. Several thousand South Ossetians are also homeless.
Rights groups said Georgian shelling of Tskhinvali during the war was indiscriminate, and that Russia bore responsibility for the ethnic cleansing of Georgians.
That Georgia's pro-Western President Mikheil Saakashvili remains in power, having faced down months of opposition protests against his rule, remains an irritant in relations.
"There are certainly those in Moscow who think that the task from last year was never accomplished, the real hardliners," said a senior European diplomat.
Russia says Saakashvili still has military designs on the rebel regions. Visiting Georgia last month, U.S. Vice President Joe Biden said there was "no military option to reintegration."
"The Georgian side doesn't have any serious potential to get back those territories. It would be completely crazy to try," said Fyodor Lukyanov, editor of Russia in Global Affairs.
If Russia had wanted to get rid of Saakashvili, it could have done so last year, he said. "... they could have 'finally resolved' the Saakashvili question then. Now, I don't see why."
But fear pervades the border zone.
"There are permanent provocations and we're scared it will all start again," said Dr. Zaira Kharyebeva, 60, in Tskhinvali. In the Georgian village of Ergneti, Giuli Kasradze, 54, stares at the torched shell of her home. "People are afraid, they are very afraid of another war," she said.
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