Serhiy Taruta, the chairman of Donetsk Regional State Administration in eastern Ukraine, has warned residents to brace for bloodshed as the deadline given to the pro-Russian protesters to lay down their weapons has passed.
The region chief said special operations to take down the separatists are underway and urged resident civilians to leave the "hot spots."
The Kiev-appointed official reminded them of Ukraine's ultimatum saying that the eastern regions were entering into anti-terror operation mode which was intended to protect the civilians.
Pouring scorn on pro-Russian protesters who stormed the state buildings, he said: "These people are willing to take the lives of others to achieve their goals. For them, the value of human life, but a means [sic]. They are terrorists, and we will not allow them to dominate our land. It is impossible to prevent the escalation of violence."
Ukraine had earlier warned that it would begin large-scale anti-terrorist operations in its eastern cities as promised, to combat the armed pro-Russian protesters who had seized state buildings.
Despite the ultimatum from the Kiev administration, there had been no sign of the separatists relinquishing their weapons until Kiev's deadline passed at 09:00 local time (06:00 GMT).
Schools and other educational institutions in Slavyansk, the flashpoint city of the Donetsk region, are said to be shut over the intensifying tensions.
Emergency talks held by the UN Security Council at Russia's request failed to yield any credible outcome to contain the unrest.