Nato made barely-concealed threats against Russia today, with alliance head Anders Fogh Rasmussen warning against making a “historic mistake” in eastern Ukraine.

“It would have grave consequences for our relationship with Russia and it would further isolate Russia internationally,” warned the cold war-era military alliance’s leader.

Ukraine’s embattled interim leaders have been waging an uphill battle to keep control of a country of 46 million after last month’s loss of Crimea to Russia.

Ukrainian forces arrested 70 people in a night-time security sweep in Kharkiv.

Acting President Oleksandr Turchynov told a rowdy session of the rump parliament that he intended to treat pro-Russian militants as “terrorists” who will face the full brunt of the law.

Deputies fought in the chamber after communist leader Petro Symonenko claimed rightwingers had played into the hands of Russia by their extreme tactics early in the crisis.

Two deputies from the fascist Svoboda party assaulted him while he was talking from the rostrum.

His supporters rallied to his defence and a brawl broke out with deputies from other parties joining in and trading punches.

Mr Symonenko said nationalists had set a precedent earlier this year by seizing public buildings.

Now, he said, armed groups were attacking people who wanted to defend their rights by peaceful means.

Kiev’s troops regained control of an administration building in Kharkiv and the security service headquarters of Donetsk.

But pro-Russian elements still held the security service building in Lugansk after releasing activists who had been accused of plotting to stage a coup.

And hundreds of militants remained inside the Donetsk administration building a day after proclaiming the creation of an independent “people’s republic” and demanding that an independence referendum be held before May 11.

Russia’s foreign ministry countered by accusing Kiev of making “military preparations that are fraught with the risk of unleashing a civil war.”

It also alleged the Kiev government was deploying private security operatives from US mercenary firm Greystone dressed up as Ukrainian special forces.