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Pro-Russian separatists claimed responsibility on Thursday for kidnapping a team of international military observers who went missing in eastern Ukraine.

The four members of the Organization for Security and Cooperation in Europe (OSCE) were detained by militants in the Donetsk region, said Vyacheslav Ponomarev, the self-appointed mayor of the separatist stronghold Slovyansk, according to The Associated Press and Interfax news agencies.

The Ukrainian Foreign Ministry confirmed the monitors - who are from Turkey, Switzerland, Estonia and Denmark - were taken by pro-Russian militants. But OSCE spokesperson Iryna Gudyma told NBC News that the organization has not been able to contact the men since they went missing on Monday and could not comment on the reports.

An armed pro-Russian separatist stands guard near Donetsk Airport on Tuesday.Alexander Ermochenko / Anadolu via Getty Images

Ponomarev said the OSCE had been warned not to travel in the region, which has been seized by militants demanding closer ties with Russia. He added the men would be released, but did not give an exact timetable.

"I addressed the OSCE mission to warn them that their people should not over the coming week travel in areas under our control. And they decided to show up anyway," he said in comments reported by the AP. "We will deal with this and then release them."

OSCE staff were deployed in Ukraine following Russia's annexation of Crimea. They have been monitoring violence between Ukraine troops and rebels across the east, as well as Sunday's presidential elections.

Russia's United Nations Ambassador Vitaly Churkin said Moscow blamed the Ukrainian government for not ensuring the safety of OSCE staff.

"We should pay attention to the fact that the Ukrainian side, which is receiving this mission, bears the most responsibility for the monitors' security," Churkin told a U.N. Security Council meeting in New York on Thursday, according to Interfax.

- Alexander Smith