Ukraine on Sunday launched an "anti-terrorist operation" in the eastern town of Slavyansk, where pro-Russian gunmen have seized police and security services buildings, Interior Minister Arsen Avakov said.
"Units from all of the country's force structures are participating. May God be with us," the minister wrote on his Facebook page.
On Saturday Avakov hit out at what he called a "display of aggression by Russia" as heavily armed separatist militants launched a series of attacks on police headquarters across the eastern rust belt.
The highly organised offensive by men in unmarked military uniforms was reminiscent of that which took place in Crimea in March, fanning fears that Russia is stoking unrest as a pretext to intervene in the neighbouring country.
Russia massed up to 40,000 troops on Ukraine's eastern border after a new pro-Western government swept to power on the back of violent winter protests against pro-Russian president Viktor Yanukovich.
Moscow on Saturday warned Kiev against using force against the militants, whose demands range from being granted greater autonomy to joining Russia.
Ukraine's security council met late Saturday for more than three hours to discuss the crisis in the east, but no decision has been announced.