Ukraine armed forces have launched action to regain control of several eastern cities held by pro-Russian militants.
It comes a day after a Ukrainian ultimatum expired for the rebels to lay down their weapons.
Dozens of Ukrainian soldiers have been brought in to Slavyansk by helicopter and armoured personnel carriers.
So far, however, there are no reports of any shots being fired.
Interim President Oleksandr Turchynov described the offensive as an “anti-terrorist operation.”
In an address to parliament he said “the aim…is to defend the citizens of Ukraine.”
Ukrainian police in several cities across the east have not put up any resistance in the face of pro-Russian militants who’ve occupied local government offices.
Writing on his Facebook page Russia’s Prime Minister Dmitry Medvedev described Ukraine as being on the “brink of civil war.”
Moscow accuses Kiev of provoking the crisis by ignoring the rights of citizens who use Russian as their first language, and has promised to protect them from attack.