The acting President of Ukraine, Olexander Turchynov, has announced the start of an “anti-terrorist operation” against pro-Russian separatists.
Addressing parliament on Tuesday, Turchynov said that the operation began in the “north of Donetsk Region” and was being conducted “stage by stage, in a responsible… manner”.
Mr Turchynov said the aim of the operation in Donetsk was to “protect Ukrainian citizens, to stop the terror, to stop the crime, to stop the attempts to tear our country apart”.
Ukrainian armoured vehicles were seen massing on Monday as separatist militants braced for an attack.
The US and Russian presidents have discussed the crisis by telephone.
Barack Obama urged Vladimir Putin to use his influence to make separatists in Donetsk and other parts of eastern Ukraine stand down.
Mr Putin denied that Russia was intervening in the crisis.
Pro-Russian rebels have seized buildings in about 10 towns and cities across Ukraine’s eastern provinces, which form the heartland of Ukraine’s heavy industry.
Thousands of Russian troops are reported to be deployed along the border, kindling fears that any crackdown on the rebels could trigger an invasion.
Russia annexed the Ukrainian province of Crimea last month, after it broke away and held a controversial referendum on self-determination.