Ukraine crisis: Crunch talks due in Geneva
Russia, the United States, the European Union and Ukraine are due to meet in Geneva to try to reduce escalating tensions over eastern Ukraine, the BBC reported.
Deep disagreement over the issue has led to the worst crisis between the US and Russia since the Cold War.
The West accuses Russia of aiding pro-Russian activists who have seized public buildings across the east.
US President Barack Obama has warned Russia against support for further action by armed pro-Russian groups.
"What I have said consistently is that each time Russia takes these kinds of steps that are designed to destabilize Ukraine and violate their sovereignty, that there are going to be consequences," he said.
Meanwhile, Ukraine's military operation against separatists has hit obstacles.
Called an "anti-terrorist" operation by the Kiev government, it started on Tuesday and is designed to dislodge pro-Russia gunmen from local authority buildings in a swathe of cities and towns in eastern Ukraine.
Pro-Russian activists want referendums on greater autonomy for the south-east or the right to join the Russian Federation.
But in several districts, Ukrainian troops met vehement opposition on Wednesday from pro-Russia supporters, who object to the new government in Kiev.
Reports from Mariupol in the south of the Donetsk region say a Ukrainian military unit was attacked with petrol bombs.
Soldiers are reported to have opened fire in response and several people were wounded, including some police.
In the city of Kramatorsk, six military vehicles were commandeered on Wednesday by gunmen, who disarmed the Ukrainian soldiers and sent some of them home on buses.
One Ukrainian officer said he had not "come to fight" and would never obey orders to shoot his "own people."
In another incident, several hundred residents of Pchyolkino, south of Sloviansk, surrounded a column of 14 Ukrainian military vehicles.
After the crowd was reinforced by pro-Russian gunmen, negotiations ensued and the troops were allowed to drive their vehicles away, but only after agreeing to surrender the magazines from their assault rifles.
Ukraine's "anti-terrorist" operation is looking more and more a non-event - or worse, an outright fiasco, reports the BBC's David Stern in Donetsk.