Nato has seen columns of Russian military equipment and Russian combat troops entering Ukraine this week, its top commander said.
“Russian tanks, Russian artillery, Russian air defence systems and Russian combat troops” were sighted, US Gen Philip Breedlove said in Bulgaria.
Russia has consistently denied sending troops into eastern Ukraine to bolster pro-Russian separatist rebels there.
However, the rebels have admitted being helped by “volunteers” from Russia.
Heavy artillery fire rocked the east Ukrainian city of Donetsk, the industrial hub held by pro-Russian separatist rebels, on Wednesday morning.
It was unclear whether the fire came from besieging government forces or the rebels themselves, or both.