Acting Ukrainian President Olexander Turchynov announced that an 'anti-terrorist operation' against pro-Russian separatists has begun in Donetsk.
He told parliament it was being conducted 'stage by stage and in a responsible manner'.
According to the BBC, Turchynov said the airbase at Kramatorsk had been 'liberated' from 'terrorists'.
Turchynov added that the aim of the operation in the east was to 'protect Ukrainian citizens, to stop the terror, to stop the crime, to stop the attempts to tear our country apart'.
Pro-Russian rebels have seized buildings in about 10 towns and cities across Ukraine's eastern provinces, its industrial heartland, the report said.
According to the report, the White House described the Ukrainian government's response as measured, and warned it was 'seriously considering' further sanctions against Russia.
Meabwhile, there were reports of gun attacks on rebel checkpoints near the Donetsk town of Sloviansk, where pro-Russian militants seized a police station and a security services building at the weekend.
A police building in Kramatorsk was also seized but the militants there have reportedly now handed back control to the police, the report added.