VIENNA, Aug 18 (Reuters) - Europe's rights and security body, the OSCE, on Monday failed to agree on sending up to 100 extra military monitors to observe a ceasefire in Georgia, but the talks were continuing, a senior official said.

The Organisation for Security and Cooperation in Europe (OSCE) had called a special session to discuss a Finnish proposal to dispatch additional monitors to observe a fragile ceasefire between Georgia and Russia.

Russian forces crushed Georgian troops sent to recapture the Moscow-backed, breakaway Georgian region of South Ossetia on Aug. 7-8. Moscow's intervention has drawn sharp Western condemnation.

The OSCE currently has eight military monitors in Georgia and was forced to close its field office in South Ossetia's main town, Tskhinvali, because of the fighting.

Finland, which currently chairs the OSCE, wanted extra monitors sent in immediately to observe a promised Russian troop withdrawal but Russia wanted the details of the mission hammered first, said Aleksi Harkonen, a senior official in Finland's delegation.

"The differences are fairly wide, (though) we have made a lot of progress today on a percentage of questions," said Harkonen. He did not elaborate. Talks were continuing at a political level to resolve the impasse, he said.

"This is a make or break situation for the OSCE," Harkonen told journalists after the session. "If we don't get an agreement now on the monitors, then there is a chance that other organisations will be quicker and do it first, and then there is no role left for the OSCE to play (in monitoring)."

Russia, Georgia and the United States are all members of the OSCE's permanent council, the organisation's decision-making body, which needs a consensus for decisions on deployment.

South Ossetia's separatist leader Eduard Kokoity said earlier on Monday he would not accept international observers in the region. (Reporting by Karin Strohecker; Editing by Jon Boyle)

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