An OSCE negotiating team in the eastern Ukrainian town of Slavyansk has secured the release, reportedly on medical grounds, of one of eight international military observers captured and deemed "prisoners of war" by pro-Russian separatists, Reuters reports.

The release came hours after the negotiators went into a meeting with the self-styled "mayor" of Slavyansk, Vyacheslav Ponomaryov, whose gunmen abducted the OSCE team along with five Ukrainian soldiers.

A spokeswoman for the separatists said the freed man is a Swede who suffers from diabetes.

Other sources identified him as "Major Johannson."

The breakaway group's spokeswoman said no other prisoners had been freed.

Ponomaryov has said he considers the OSCE team to be "prisoners of war."

To recall, a group of unidentified individuals seized a bus carrying OSCE observers at the entrance to Slavyansk (Donetsk region) early on Friday and convoyed it to the town.