The Ukrainian government says it has regained control of the city hall in Mariupol, eastern Ukraine, from pro-Russian separatists.
Several people are said to have been hurt during the overnight operation in the city, where three pro-Russian protesters were recently shot dead.
Reports are coming in of a Ukrainian army operation outside the separatist stronghold of Sloviansk.
Separatists are occupying key buildings in at least a dozen eastern towns
In another development, the funeral is taking place of a pro-Ukrainian politician, Volodymyr Rybak, who was abducted and killed in the 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.