The Organisation for Security and Cooperation in Europe says it is working to secure the release of a number of Special Monitoring Mission members who had been detained in eastern Ukraine.
"The OSCE is extremely concerned that a number of SMM national mission members have been deprived of their liberty in Donetsk and Luhansk. The OSCE is using all available channels to facilitate the release of its staff," its media office said in response to a query, giving no more details.
In an address to the 157-member body on Friday, the United Kingdom's deputy ambassador to the Vienna-based OSCE had criticised Russia for refusing to extend the SMM's mission in Ukraine beyond March.
"And now we have received alarming reports that Russia's proxies in Donbas are threatening Mission staff, equipment and premises and that Russian forces have taken SMM staff members captive," Deirdre Brown had said in her address, which was released by the UK government.
The OSCE said in March it had removed nearly 500 international mission members from Ukraine.
The OSCE media office said on Saturday the SMM continued to assist remaining staff in Ukraine in trying to relocate to safer areas.
"Contacts with national mission members continue on a daily basis, including in order to ascertain their whereabouts and assist them, to the extent possible, should they decide to re-locate," it added.