Ukraine's increasingly desperate interim government has reintroduced military conscription as separatists tighten their grip on the country's east. Further government buildings have been seized in several cities.
Interim President Oleksandr Turchynov signed a decree requiring men aged 18-25 to once again perform compulsory military service.
A statement issued by the president's office said conscription had become necessary to counter "the deteriorating situation in the east and the south."
He said Ukraine's military needed to protect the east from the "naked aggression" of pro-Russian separatists who were threatening the country's "territorial integrity."
The decree, which is expected to take effect either this month or next, came just half a year after conscription was scrapped under the former Russian-allied government.
Russia's foreign ministry responded, warning that any effort by Ukraine to intensify its military operation against ethnic Russians in the region would have "catastrophic consequences."
The move came a day after Turchynov declared that Ukraine's armed forces were on "full combat alert." He warned that Ukraine was losing control of its more heavily ethnic Russian regions.
His concerns were further realized on Thursday when pro-Russian separatists took control of another government building in the disputed eastern city of Donetsk.
Around 300 masked protesters hurled stones, petrol bombs and firecrackers at outnumbered riot police protecting the prosecutor's office, before storming the six-storey building.
Several police officers were injured. Some were stripped of their batons and shields.
Ukrainian media reported that an administrative building was also overrun in the town of Horlivka, as was a police station in Krasnoarmiysk.
Separatists controlling dozens of buildings are attempting to force the east's secession from Ukraine. Some are hoping to follow in the footsteps of the Crimean Peninsula and hold a referendum on joining the Russian Federation.
Western nations have joined Kyiv in accusing Russia of orchestrating the unrest in order to justify a military intervention. However Moscow has strenuously denied the allegations.
ccp/jlw (AFP, AP, Reuters, dpa)