Pro-Russian militants seized a bus carrying international military observers and Ukrainian Army soldiers on Friday, according to the Ukrainian Interior Ministry, as Russia called on Kiev to halt all military action in eastern Ukraine.

The Ukrainian army has launched an assault on pro-Russian forces who have seized control of the flashpoint town of Slavyansk.

The bus, on the road to Slavyansk, was carrying seven officials from the Organization for Security and Cooperation in Europe, five Ukrainian soldiers and a driver, all of whom were taken hostage by “terrorists,” according to the ministry. They are reportedly being held in the offices of the state security service building, which pro-Russian forces seized earlier this month.

“Talks with the pro-Russian forces are going on now. They refuse to let the hostages go, saying they need to talk to the competent authorities in Russia,” the ministry said.

The local pro-Russian militia leader told Moscow-based media outlet RIA Novosti that the bus had been stopped because of Ukrainian military personnel onboard.

“We are checking identities and background of OSCE observers and officers,” said Vyacheslav Ponomarev, adding that there was reason to believe there was a Kiev spy among the detained officials.

A pro-Russian armed man in military fatigues stands guard outside the security service (SBU) regional building which was seized earlier this month by pro-Russian separatists in the eastern Ukrainian city of Slavyansk, April 25, 2014. (photo credit: AFP/Kirill Kudryavtsev)

A pro-Russian armed man in military fatigues stands guard outside the security service (SBU) regional building which was seized earlier this month by pro-Russian separatists in the eastern Ukrainian city of Slavyansk, April 25, 2014. (photo credit: AFP/Kirill Kudryavtsev)

Earlier in the day, the Russian Foreign Ministry issued a statement saying, “Russia calls for an immediate stop to all military action and violence, for the retreat of troops and the full implementation of the Geneva accord by Ukraine.”

“This will allow a start to de-escalation,” the ministry said.

It said Russia supports the clear implementation of the Geneva accord which it signed along with Ukraine and Western powers last week, and which calls for all illegal armed groups to disarm and all occupied buildings in Ukraine to be evacuated.

Foreign Minister Sergey Lavrov said earlier Friday following a phone call with his German counterpart Frank-Walter Steinmeier that he supported “the important role of the OSCE in the dialogue between the authorities in Kiev” and the rebels in eastern Ukraine.

AFP contributed to this report.