Pro-Russian rebels holding a group of international OSCE observers in eastern Ukraine on Saturday accused them of being “NATO spies” and vowed to continue detaining them.

“Yesterday, we arrested some NATO spies… they will be exchanged for our own prisoners. I don’t see any other way they will be freed,” Denis Pushilin, the head of the insurgents’ self-declared Donetsk Republic, told reporters.

Pushilin was speaking in front of the SBU security services building in rebel-held Slavyansk, where the OSCE team was being held.

The town has become the epicentre of tensions between pro-Russian protesters and Ukrainian authorities in the eastern part of the country where pro-Kremlin rebels have seized a string of towns.

Late Friday, German Defence Minister Ursula von der Leyen confirmed that pro-Russian separatists had arrested 13 mission members, including the observers, their interpreter and driver. Four of the team are Germans, including three members of the German military.