The Ukrainian Security Service (SBU) says it has detained an alleged mercenary suspected of having fought on the side of pro-Russian separatists in the country's east.
The SBU said on April 2 that a 26-year-old Kharkiv resident had been detained after returning to the eastern Ukrainian city from Russia's Rostov region.
Investigators believe the man received money to fight alongside separatists in the Donetsk region, which borders Kharkiv province.
They said preliminary investigations revealed he was wounded last year and treated in the Rostov region.
The suspect is facing charges of threatening Ukraine's territorial integrity and could be imprisoned for five years if convicted.
More than 6,000 people have been killed in eastern Ukraine in fighting between rebels and pro-government forces since April 2014.
Kyiv and the West accuse Russia of sending troops and arms to aid the separatists, a charge Moscow denies.