Ukrainian citizens account for a mere 15-20% of the militants who are fighting in illegal armed formations in Donbas, according to the anti-terrorist operation (ATO) staff.

"According to reconnaissance data, the share of people with Ukrainian citizenship in illegal armed formations as of March 12 was about 15-20%, the remaining are Russian mercenaries," ATO spokesman Andriy Lysenko said at a briefing in Kyiv on Friday.

The enemy is continuing to beef up its formations in the ATO zone in all major directions along the line of contact, to regroup its forces and military hardware, including air defense missile systems he said.

"Operational reports say that intensive military training of units of the Russian-terrorist forces goes on. Teams of instructors are staffed mostly with Russian citizens, both serving officers and mercenaries with profound experience of servicing in the Armed Forces of the Russian Federation and participation in military operations," Lysenko added.