South Ossetia has been trying to gain independence since the early 1990s
The OSCE has condemned the brief detention of two unarmed observers by separatist forces in Georgia's breakaway region of South Ossetia.
The OSCE's chairperson-in-office, Greek Foreign Minister Dora Bakoyannis, welcomed their release but described the action as "wholly unacceptable".
Earlier, South Ossetia's leader said they were held for "illegally crossing the Georgian-South Ossetian border".
The monitors are overseeing a ceasefire agreement between Georgia and Russia.
Georgia and Russia fought a brief but intense war last August, during which Georgia's attempts to regain control of South Ossetia and its other breakaway region of Abkhazia were repelled by Russian forces.
Both regions have been trying to gain formal independence since breaking away in the early 1990s. Russia has now recognised them as independent - a move condemned by Western nations.
In a statement, the Organisation for Security and Co-operation in Europe said the two military monitoring officers - one Hungarian and the other Turkish - were detained on Tuesday morning near the village of Nikozi, south of the administrative boundary between Georgia and South Ossetia.
They were released after about two-and-a-half hours, during which time they were taken by the separatist authorities to the South Ossetian capital, Tskhinvali.
"I am relieved the two military monitoring officers have been freed, but their detention was wholly unacceptable and unnecessarily added to tensions in the region," Ms Bakoyannis said.
"It is crucial that all parties act constructively to contribute to security," she added.
Ms Bakoyannis said the incident underlined the importance of defining a framework that would allow the unhindered monitoring of the area.
OSCE military observers have been based in Georgia since 1992, but following last year's conflict, the South Ossetian authorities have denied them access. Russia, which has thousands of troops in the territory, has also objected to their presence.
Earlier, the head of South Ossetia's separatist administration, Eduard Kokoity, said the observers had illegally driven into South Ossetian territory south of Tskhinvali and had been detained by border guards.
"The actions of the OSCE observers have a provocative nature," he told Russia's Interfax news agency.
Two OSCE monitors were detained for three hours by separatist forces in February after allegedly straying into South Ossetia.