Russian military equipment and Russian combat troops have entered Ukraine this week, NATO commander said, BBC reports.
US General Philip Breedlove said: "’Russian tanks, Russian artillery, Russian air defence systems and Russian combat troops’ were sighted.”
Russia's defence ministry denied that its troops were in eastern Ukraine to help pro-Russian separatists 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.
There were also reports of fighting near the rebel-held city of Luhansk. One Ukrainian soldier was killed and another injured north of Luhansk, when rebels fired on government positions near the village of Schastya, Ukrainian security forces said.