The confirmation comes after days of reports from Ukrainian and European officials of tanks crossing into eastern Ukraine in scenes reminiscent of Russia’s invasion of Crimea.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.
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.
More than 4,000 people have died since government forces moved in April to put down an armed insurrection by the rebels in the two regions, which border Russia.
A fragile ceasefire was agreed in Minsk on 5 September, although hundreds of people have been killed since then.
German Chancellor Angela Merkel said on Tuesday her government was "not satisfied" with the lack of progress in implementing the Minsk agreement but added that there were no plans at present for further economic sanctions on Russia over its involvement in Ukraine.
"Further economic sanctions are not planned at the moment, we are focusing on the winter and the humanitarian situation there and how to get a real ceasefire," she said.
The Organization for Security and Cooperation in Europe (OSCE) said there was a "rising" risk that the conflict would get worse.
"The level of violence in eastern Ukraine and the risk of further escalation remain high and are rising," OSCE representative Michael Bociurkiw told reporters in Kiev. "More than two months after the signing of the Minsk protocol memorandum, the firing has not ceased."