Ukraine's acting president Oleksander Turchynov has called on Russia to pull troops back from their border, stop interfering in Ukraine's internal affairs and end its "permanent threats and blackmail".

Mr Turchynov broadcast his brief address to the nation after Russia said it had begun military exercises near the border in response to escalation by Ukraine.

He told the nation that Russia was "demonstratively concentrating armed forces" on the frontier.

"We will not give into terrorist threats and will go on taking measures to protect the lives of our citizens," Mr Turchynov said.

"We demand Russia end its interference in the internal affairs of Ukraine, its permanent threats and blackmail, and withdraw its troops from the eastern border of our country."

Ukrainian Interior Ministry and Defence Ministry forces were in action against pro-Russian separatistrebels near the eastern city of Slaviansk earlier today and said they killed "up to five".

Russian President Vladimir Putin, who has promised to protect Russian-speakers in Ukraine, said there would be consequences if the authorities in Kiev used the army against their own people.

"If Kiev really began to use the army against the country's population ... that is a very serious crime against its own people," Mr Putin said, calling the current pro-West authorities a "junta".

"That is simply a punitive measure that will without question have consequences ... including for our inter-governmental relations."

Ukraine's military launched an assault on Slaviansk sending in armoured vehicles and a helicopter.

Several armoured personnel carriers drove past an abandoned rebel roadblock in flames to take up position at the entry to the town.

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