The Organization for Security and Cooperation in Europe (OSCE) says it has lost contact with a second team of observers in separatist-controlled eastern Ukraine, Reuters reports.
The pan-European rights and security body said it could not get in touch with some members of its Luhansk-based team.
The OSCE lost contact with a first team of four civilian observers east of Donetsk on the evening of May 26.
The OSCE has so far not confirmed a claim by Aleksei Chmilenko, a leader of the self-proclaimed Luhansk People's Republic, who said on May 30 that the team had been released after their documents were checked.
Chmilenko did not say when the monitors were released.
Separately, a spokesman for the separatists, Volodymyr Inohorodskikh, said on May 30 that the OSCE team had been prevented -- for their own safety -- from leaving overnight during a separatist-imposed curfew.