Commandos have moved on the separatist stronghold of Sloviansk in eastern Ukraine days after a new "anti-terrorist" operation was announced.A number of casualties were reported among the separatists as the commandos, backed by armour, cleared barricades near the town.
Separatists are occupying key buildings in at least a dozen eastern towns.Russian President Vladimir Putin warned of "consequences" if Kiev used the army against its own people.
Speaking on Russian TV as news of the Sloviansk operation was coming in, Mr Putin said: "If the regime in Kiev has begun using the army against the population inside the country, then this is undoubtedly a very serious crime."It appears that pro-government forces have regained control of the city hall in another eastern town, Mariupol.
In another development, the funeral is taking place of a pro-Ukrainian politician, Volodymyr Rybak, who was abducted and killed in Donetsk region.
Meanwhile, US President Barack Obama has accused Russia of flouting a deal on Ukraine reached in Geneva last week, under which illegal armed groups, including those who have seized public buildings, would return home.Mr Obama told a news conference in Japan that Moscow had failed to halt actions by militants in the region and warned that the US had further sanctions against Russia "teed up".
A contingent of US troops has begun landing in Poland for military exercises amid concerns among Nato's eastern members about Russian intentions.Moscow has said it will respond to any attack on its interests in Ukraine.Unrest began in Ukraine last November over whether the country should look towards Moscow or the West.