Pro-Russian separatists in the eastern Ukrainian city of Sloviansk have shot down two Ukrainian military helicopters, during a “counterterrorism” operation ordered by Kiev to clear the area of insurgents.

According to the BBC, a pilot and another soldier have been killed, four pro-Russian militiamen arrested and 10 separatist checkpoints seized during the operation so far.

In a statement, the Ukrainian government announced it had retaken half of the city, and the head of Ukraine’s National Guard said his units had “practically cleared Sloviansk from terrorists”.

Ukraine’s Interior Minister Arsen Avakov described the fighting in the Sloviansk-Kramatorsk region, which began at 04:30 local time.

“The terrorists opened fire with heavy weapons against Ukrainian special units,” he said. “A real battle with professional mercenaries is going on.”

He accused rebels of using civilians as human shields by hiding behind residential buildings. Other reports claimed pro-Russian forces had disguised themselves as Ukrainian riot police and were directing sniper fire towards advancing military units.

Meanwhile, Russian’s foreign ministry decried what it termed Kiev’s use of force on its own people. Moscow had previously warned of “catastrophic consequences” if Ukrainian armed forces moved in on eastern Ukraine, a hotbed of pro-Russian insurgents, in what was taken by many analysts as a veiled warning of a Russian military response.

The clashes come amid a seemingly unstoppable momentum of escalation in the country, with Ukraine’s acting President Olexandr Turchynov reinstating military conscription in an attempt to reassert Kiev’s control over eastern Ukraine, much of which he admitted it was currently powerless to retake from pro-Russian forces.