The Organization for Security and Cooperation in Europe (OSCE) is making contacts with pro-Russian militants in eastern Ukraine for freeing detained military observers, said German Foreign Minister Frank-Walter Steinmeier on Saturday.

"The OSCE endeavors on site contacts and talks with the ones who have detained the observers group and to achieve a release. We will give our full supports," said Steinmeier in a recorded statement sent to Xinhua.

He said a crisis management team was set up Friday in German Foreign Ministry and he had already contacted with officials from OSCE, Ukraine and Russia.

Earlier on Saturday, German media reported that the Vienna-based OSCE had sent a negotiation team to Ukraine for freeing a group of observers detained by pro-Russian fighters in east Ukrainian city of Sloviansk on Friday, citing unnamed German government source.

Among the detained group members, four were Germans, including three observers and one interpreter, according to German Defence Minister Ursula von der Leyen. The others include a Czech, a Dane and a Swede.

The group were sent to Ukraine last month to monitor political and security situation in the country following pro-Russia protests in its eastern region.

Pro-Russian fighters claimed that a Ukrainian spy was traveling with the group. A spokesperson of the fighters said on Saturday that they have not ruled out releasing the detainees in exchange for the release of their own men.