At least 6,000 military personnel of the Russian Federation are currently fighting in Donbas, the Ukrainian State Security Service (SBU) has reported.

"Today, according to our data, around 6,000 Russian military men remain in the territory of Donbas," head of the SBU office Oleksandr Tkachuk said at a briefing in Kyiv on Tuesday.

"Russian servicemen are constantly rotated in the territory of Ukraine not controlled by the state authorities. Their numbers have decreased slightly, but not significantly," he said.

Tkachuk has not specified the exact number of Russian citizens staying in Donbas.

He stressed that the total number of members of illegal armed groups, who are located in Donbas, is about 40,000.

Tkachuk estimated that more than 1,600 Russian servicemen have been killed in eastern Ukraine since Russia illegally annexed Ukraine in early 2014.