One of two Russian servicemen Ukraine says it captured while fighting with separatists in eastern Ukraine admitted on a video circulating on Monday that he had been part of a Russian special forces spying mission.

Moscow, which has acknowledged in the past that Russian national "volunteers" might be fighting for the separatists, repeated its denial that any regular Russian troops were involved in the campaign which has cost over 6,100 lives.

The wounded soldier, who identified himself as a sergeant in the 'spetsnaz' from the central Russian city of Togliatti, said he was part of a 14-member group operating near the big border town of Luhansk.

The video could not be independently verified but if the soldier, who spoke from a hospital bed, did turn out to be a serving Russian officer it would support Kiev's accusations of direct Russian military involvement in the separatist conflict.

The Ukrainian military said on Sunday that the two Russian soldiers had been captured during a battle with rebels at Shchastya, near Luhansk, in which a Ukrainian soldier was killed.

Though a tenuous ceasefire declared last February stemmed large-scale loss of life in a conflict in which more than 6,100 people have been killed, skirmishes have continued between government forces and the separatists, with casualties on both sides.

The soldier on the video, who gave his name as Alexander Alexandrov, said his group was on a mission near the line of separation between the warring sides "to carry out observation".

"We were discovered. I was wounded in the leg as I tried to get away. There were 14 members in the group. We've been here 4-5 days," he said on the video.

Apart from the Ukrainian soldier killed near Shchastya, two other Ukrainians were killed at the weekend as a result of a mortar attack near the town of Svitlodarsk, northeast of separatist-controlled Donetsk.