The Ukrainian military has accused Russia of attacking its forces as fighting in eastern Ukraine spreads towards the Russian border.
Military spokesman Andriy Lysenko said regular Russian units had attacked in the Luhansk region, much of which is under pro-Russian rebel control.
Heavy fighting was under way near the town of Slovyanoserbsk, he said.
Russia denies sending troops into Ukraine but acknowledges "volunteers" are fighting for the rebels.
There has been no independent confirmation of the latest Ukrainian allegation.
Meanwhile Ukrainian President Petro Poroshenko cut short his visit to the World Economic Forum in Davos, Switzerland, to return home in view of the "worsening situation" in the east, his spokesman said.
The Vienna-based Organization for Security and Co-operation in Europe (OSCE), which has monitors deployed in Ukraine, has called for an immediate ceasefire.
More than 4,800 people have been killed and some 1.2 million have fled since the rebels took control of parts of Luhansk and Donetsk in April.
Fighting flared up again last week as the rebels fought the Ukrainian government for control of the ruined airport at Donetsk.
Two civilians were killed in the rebel-held city on Tuesday when a shell exploded at a bus stop.