The world's largest security body on Sunday called for the "immediate release" of four of its Ukrainian members detained in pro-Russian separatist territories in the country's east.
The Organization for Security and Cooperation in Europe (OSCE) evacuated many of its staff from the country after Russia's invasion of Ukraine on February 24.
Many had been in the east to observe a cease-fire after Russia's 2014 annexation of the Crimean Peninsula and several Ukrainian members have stayed on.
Polish Foreign Minister Zbigniew Rau, whose country currently chairs the OSCE, said the detention of four members "for engaging in administrative activities that fall within their official functions as OSCE staff" was "unacceptable."
"We call for their immediate release," he said. "They have been held without charge for a period of time now and the OSCE and their families have not been sufficiently informed of the situation."
Earlier Sunday, the OSCE said on Twitter it was "extremely concerned" that several members had been "deprived of their liberty," in line with Russian media reports they had been arrested.
It added that it was "using all available channels to facilitate their release."
The security services of the Lugansk separatists said this month they had arrested two members of the OSCE mission, according to Russia's TASS news agency.
TASS said one of them had "confessed" to passing "confidential military information to representatives of foreign special services" and that a high treason investigation had been opened against him.
The Donetsk separatist prosecutor's office confirmed he had opened espionage probes against several OSCE employees suspected of having collected and transmitted "information constituting state secrets" to the Ukrainian secret services.
The prosecutor's office said this included videos, revealing the position of separatist military units.
On Sunday, the U.S. ambassador to the OSCE, Michael Carpenter, also called for the members of the body's Ukraine Special Monitoring Mission (SMM) to be released.
"Russia's lies claiming Ukrainian OSCE_SMM staff spied for the Ukrainian government are reprehensible," he said on Twitter.
The OSCE international observation mission in Ukraine was not renewed at the end of March, after Moscow blocked the proposal.
Authorities in Ukraine's separatist regions have given OSCE staff until the end of April to cease all activities.
The Vienna-based OSCE has 57 member states on three continents — including Russia, Ukraine and the United States.