A Moldovan was arrested by prosecutors and SIS officers on suspicion he fought as a mercenary by the separatists in Donetsk.
The 42-year-old man , is originally from Transnistrian region and was a member of military formations "Sparta", led by one of the leaders of the Ukrainian region.
The alleged mercenary was being investigated by the Information and Security officers since 2015.
The man was detained for 72 hours, and prosecutors will ask for warrant arrest.
If found guilty, he risks seven years imprisonment.