The Ukrainian Security Forces have launched an anti-terrorist operation to drive out pro-Russian separatists from government buildings and restore authority in the east of the country.
In an address to parliament, Ukraine President Olexsandr Turchynov confirmed that an operation has started in the east of the Donetsk region, and will be conducted 'step-by-step', in a responsible manner.
"The aim of this action, I emphasise it once again, is to protect Ukraine's citizens, to stop terror, to stop criminality and stop attempts to tear Ukraine apart," Turchynov said.
Disembarking from two helicopters, the Special Forces regained control of an occupied airfield in the city of Kramatorsk, amid a scuffle and demands for a referendum by the insurgents.
A Ukrainian state security service spokesperson said a similar operation is also underway in the nearby town of Slaviansk, which is completely surrounded by barricades and checkpoints manned by pro-Russian militia.
The military crackdown comes a day after the activists ignored the government's deadline to lay down their weapons and leave the administrative buildings under amnesty by Monday.
The activists have either seized or barricaded state buildings in at least ten eastern Ukrainian cities over the weekend, demanding greater autonomy from the new Ukrainian government and closer ties with Russia.
Meanwhile, Russian President Vladimir Putin said the escalation of the crisis had put Ukraine 'on the verge of civil war'.
Moscow had already warned Kiev against the use of force against Russian speakers, saying it 'would undermine the potential for cooperation', including talks scheduled to be held on Thursday between Russia, Ukraine, US and the EU in Geneva, Switzerland.
Russian Foreign Minister Sergei Lavrov said: "You can't send tanks against your own citizens and at the same time hold talks."
The standoff has raised fears that Russia might intervene militarily to 'protect' Russian speakers. It is believed to have tens of thousands of troops on the border with eastern Ukraine.