“Near the entrance to Slavyansk, unknown persons stopped the bus with 13 passengers including seven OSCE representatives, five representatives of the armed forces of Ukraine and the driver of the vehicle,” the ministry said in a statement.

It said the passengers were taken to the occupied security services building in the town and the rebels were demanding to speak with “the competent authorities of the Russian Federation”.

The Vienna headquarters of the Organisation for Security and Cooperation in Europe did not immediately confirm the information to AFP, but did say that all the observers in its main mission on the ground in Ukraine were accounted for.

It explained though, that a smaller, unarmed OSCE military verification mission was also in the country, under German command.

The self-styled mayor of Slavyansk, Vyacheslav Ponomaryov, told reporters that there was a “spy” on board the bus.

“Inside there was a person who worked for the Ukrainian army chief of staff. He’s a spy,” Ponomaryov said.

When asked if they had been arrested, he replied: “I don’t know. It’s my staff who are dealing with it.”

Their abduction sparked swift international outrage, with Swedish Foreign Minister Carl Bildt demanding they be “released immediately”.

Bildt added he was “extremely concerned” and that one of the observers held was a Swede.

burs-rmb/ric/hmn