German Defense Minister Ursula von der Leyen said late Friday that four Germans were among the international military observers who had been reportedly detained in eastern Ukraine.

Earlier in the day, the Ukrainian Foreign Ministry said Kiev had lost contact with a military mission of the Organization for Security and Cooperation in Europe (OSCE) deployed in Ukraine's eastern region of Donetsk.

A spokesman of the ministry said preliminary information suggested the OSCE observers might have been kidnapped by pro-Russia protesters who took control of several cities in Donetsk.

Ursula von der Leyen said increased clues were showing that a 13-member OSCE group had been detained, adding that three German observers and their interpreter were on the mission, according to the Bild newspaper.

The minister said the German Foreign Ministry had set up a crisis management team and the German government would use all its diplomatic channels to establish contact with the missing people again.

A leader of pro-Russia protesters in Sloviansk told the Bild Friday that they seized a bus of OSCE observers in the east Ukrainian city, including five Ukrainian soldiers who were accused of spying.

"They are not hostages but prisoners of war," he claimed.

The OSCE said the military observers came from five European countries: three are from Germany, and one each from the Czech Republic, Denmark, Poland and Sweden.

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

Read more in Special Coverage: