The Russian foreign ministry has warned Ukraine against a military build-up in the east of the country as tensions continue to escalate.

Moscow has said Kiev's latest attempt to strengthen its forces in the eastern cities, allegedly with the help of US mercenaries, will potentially lead to a "civil war".

In a statement, the foreign ministry said: "We are particularly concerned that the operation involves some 150 American mercenaries from a private company, Greystone, dressed in the uniform of the [Ukrainian] special police unit task Sokol.

"Organisers and participants of such incitement are assuming a huge responsibility for threatening upon the rights, freedoms and lives of Ukrainian citizens as well as the stability of Ukraine.

"We call for immediate cease of any military preparations that could lead to unleashing the civil war."

Moscow's warning came hours after Ukraine said it was deploying special forces in its eastern cities to quell the snowballing protests.

"These special forces are ready to solve operational problems without the regard to local peculiarities. I urge all the hotheads now to defer from criticism and panic, and help the police keep the situation under control," said Ukraine's interior minister Arsen Avakov.

In a dramatic turn of events after Crimea's breakaway from Ukraine, the eastern Ukrainian cities of Donetsk, Lugansk and Kharkov have been witnessing rallies and unrest, demanding a Crimea-style referendum.

Shortly after pro-Russian protesters occupied state buildings in Donetsk and declared independence, demonstrators in Kharkov proclaimed a "People's Republic of Kharkov".

Clashes broke out when security forces attempted to control the situation in Kharkov while trying to flush out the anti-Kiev campaigners from government buildings.