Ukrainian troops have entered the eastern town of Kramatorsk a day after an operation began to recapture areas seized by anti-government separatists.
But they were blocked by civilians and the situation is unclear, amid reports that some may have abandoned their vehicles or even changed sides.
In the city of Donetsk, which has seen weeks of unrest, pro-Russian gunmen seized control of the mayor’s office.
Meanwhile Nato announced it was beefing up its eastern members’ defences.
As tensions rose, Ukraine’s acting Defence Minister, Mykhailo Koval, was said to be travelling to the east of the country to monitor the progress of the “anti-terrorist operation” announced by acting President Olexander Turchynov the previous day.
The interim Prime Minister, Arseniy Yatsenyuk, called on Russia to “stop supporting terrorists in Ukraine”.
The crisis escalated in eastern parts of Ukraine this month after pro-Russian rebels seized buildings in about 10 towns and cities, demanding greater autonomy or referendums on secession.
Tens of thousands of Russian soldiers are believed to have massed on Ukraine’s borders since Russia took control of the Ukrainian region of Crimea last month, following a controversial referendum on self-determination.