Kiev (AFP) - The United Nations said Wednesday that one of its staff members had been captured by pro-Russian rebels in Ukraine's separatist east.

The UN is carrying out a humanitarian mission in the war-torn republics of Lugansk and Donetsk.

The world body's office in Kiev said it had mobilised all channels to ensure its unnamed staff member's immediate and unconditional release.

"The United Nations is deeply concerned about the fact that one of its staff members is being held captive in Donetsk," the UN said in a statement.

It added that is was aware that staff member was being "well treated" but provided no details about the captive's nationality or when and under what circumstances the capture occurred.

The authorities of the self-proclaimed Donetsk People's Republic (DNR) confirmed the detention but identified the captive as a member of Ukraine's SBU security service who was suspected of involvement in fighting against the insurgents.

"During his detention, he said that he was a member of the UN mission in Donetsk," the rebels said a statement.

"At the same time, (the man) was not officially registered as a member of the UN mission with the DNR authorities."

Nearly 9,200 people have died and more than 1.5 million been driven from their homes since a pro-Moscow revolt broke out in the former Soviet republic's industrial heartland in April 2014.

A series of periodic truce deals in 2015 have lessened the violence, which Kiev and its Western allies accuse Moscow of stirring and backing, a charge that Russia denies.

However on Sunday, the European Union criticised the "unprecedented level of violence" in eastern Ukraine after international peace monitors came under fire.

The Organisation for Security and Co-operation in Europe (OSCE) said Saturday that several monitors carrying out an observation mission had come under fire 50 kilometres (30 miles) south of the insurgents' de facto capital Donetsk.

In another incident, on Thursday, an OSCE monitor was threatened at gunpoint by a rebel, forcing the patrol to leave a checkpoint they had intended to pass, the group said.