(Updates with soldiers' release)

By Margarita Antidze

TBILISI, July 8 (Reuters) - Four Georgian soldiers detained in Georgia's breakaway region of South Ossetia were freed on Tuesday after President Mikheil Saakashvili threatened to send police to release them.

Russian-backed separatists in South Ossetia -- a tiny sliver of land high up in the Caucasus mountains -- detained the soldiers on Monday, raising tension a day before U.S. Secretary of State Condoleezza Rice visits Tbilisi.

"Yes, they have been released," Mamuka Kurashvili, commander of the Georgian peacekeeping battalion in South Ossetia, told Reuters.

Saakashvili had earlier ordered police to prepare an operation to free them. The separatists said the Georgian soldiers were scouting targets for a possible artillery attack.

Tension has been rising in South Ossetia and Georgia's other breakaway region of Abkhazia, both the focus of friction between the pro-Western government in Tbilisi and neighbouring Russia, which supports the separatists.

Two people were killed in South Ossetia last week in one of the most intense exchanges of fire in months between the separatists and Georgian forces. In Abkhazia, on Georgia's Black Sea coast, four people were killed when a bomb exploded in a cafe on Sunday.

The two regions broke away from Tbilisi's rule after wars in the 1990s.

Russia, which has peacekeepers in both regions, says Saakashvili wants to bring the regions back under central rule by force and the local populations do not want to be part of Georgia.

Georgia, a staunch U.S. ally seeking NATO membership, says Moscow wants to annex the regions. It accuses Russia of using the separatists to cause tension.

Rice will arrive in Tbilisi on Wednesday. Speaking in the Czech Republic on Tuesday, she said Russian behaviour towards Georgia had aggravated tension and urged Moscow and Tbilisi to avoid provocative action. (Reporting by Margarita Antidze; Writing by Guy Faulconbridge and Christian Lowe; Editing by Andrew Dobbie)

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