A German-led group of international military monitors was detained in eastern Ukraine as being suspected NATO spies, a pro-Russian insurgency leader confirmed Saturday.
The eight-member team was reported missing on Friday, German government officials said.
Vyacheslav Ponomarev, self-proclaimed people's mayor of Slovyansk, described the detained observers as "captives" and said that they were officers from NATO member states because of "maps containing information about the locations of Ukrainian checkpoints" found with them.
Ponomarev said the members of the German group could be released in exchange for jailed pro-Russian activists.
In a statement released Saturday, Russia's Foreign Ministry said it was taking "all measures to resolve the situation," but blamed the authorities in Kiev for failing to secure the safety of the team.
U.S. State Department spokeswoman Jen Psaki condemned the reported hostage taking as "repressive and cowardly."
"We are deeply concerned about reports that unidentified gunmen have abducted a Vienna Document inspection team that's, of course, part of the OSCE and their Ukrainian escorts in the town of Slaviansk," Psaki said Friday.
The military verification team is not part of a bigger special monitoring mission in Ukraine of the Vienna-based intergovernmental Organization for Security and Co-operation (OSCE) in Europe. The German-led team, which is in the Ukraine under an OSCE mechanism, is composed of seven military observers and one translator, and includes four Germans, one Dane, one Polish citizen, one Swede and one Czech.
"They are with us in Slaviansk," the de facto mayor of the city, Vyacheslav Ponomaryov, told Reuters in front of the seized security service building where, according to the Ukrainian government, the observers were being held. "What the situation was I do not know," he said. "It was reported to me that among them (the observers) was an employee of Kiev's secret military staff."
"People who come here as observers bringing with them a real spy: It's not appropriate."
Russian military forces have been “pulsing the border” and making a “run toward Ukraine” — but pulling up well short — in their “training exercise," U.S. officials told NBC News.
Secretary of State John Kerry on Thursday issued a stern warning to Russia to take concrete measures to de-escalate the crisis in Ukraine or face further sanctions.
— Catherine Chomiak and Becky Bratu
Reuters contributed to this report.