Military officials in Ukraine say at least four battalions of Russian soldiers are fighting alongside separatists in the East of the country.
Earlier pro-Russian rebels claimed to have seized the international airport in the city of Luhansa – as Ukrainian troops were reported to have withdrawn from the area.
It comes as Russian and Ukrainian officials meet with rebel leaders in Minsk for talks.
Ahead of the meeting the Russian Foreign Minister Sergei Lavrov urged all sides to make an "immediate ceasefire" their priority.
