Kiev has accused Moscow of seeking to trigger a "third world war" as military tensions soared in east Ukraine and US President Barack Obama led a diplomatic charge against Russia.

Ukraine's military on Friday mounted a second phase of an aggressive operation to regain control of the besieged city of Slovyansk even as one of its helicopters was blown up after being hit with rocket fire at a base outside the city.

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Officials in Kiev said a rocket-propelled grenade blew up the military helicopter sitting on the tarmac at a base near the eastern town of Kramatorsk. The cause of the explosion has not yet been verified.

The incident came amid an offensive to drive pro-Kremlin gunmen out of eastern Ukraine.

Kiev announced its forces were seeking to "blockade" rebels inside the flashpoint town of Slovyansk to prevent more of what it calls "terrorists" arriving.

Al Jazeera's Hoda Abdel-Hamid, reporting from Donetsk said Slovyansk was still accessible despite claims it had been surrounded.

"We haven't seen any sign of that yet on the ground. Based on what we have seen over the past ten days it seems they have had some false starts it is difficult for them to apply on the ground," she said.

Moscow responded to the military action by saying Kiev's crackdown in the east is a bloody crime and Foreign Minister Sergey Lavrov warned Ukraine that it would face justice for waging a war against its own people.

Russian troops massed on the border with Ukraine have also relaunched military exercises in response to the crackdown.

The situation in Ukraine has become increasingly militaristic, despite an apparent deal being reached in Geneva whereby all sides agreed to an amnesty for protesters who gave up occupied buildings and a blanket disarming of unauthorised groups.

Global conflict

On Friday Latvia welcomed American troops on its soil, part of a US force of 600 sent to the region to reassure the Baltic states amid concern over Russia's actions in Ukraine.

Ukrainian Prime Minister Arseniy Yatsenyuk accused Russia of trying to spark a global conflict.

"The world hasn't forgotten the Second World War and Russia wants to start a third world war," he said.

"Russia's support for the terrorists in Ukraine constitutes an international crime and we call on the international community to unite against the Russian aggression."

US Secretary of State John Kerry said the Kremlin was making an "expensive mistake" by meddling in Ukraine, stressing that further Western sanctions on Moscow were poised to be unveiled.

Moscow, Kerry said, was trying to "actively sabotage the democratic process through gross external intimidation".

"Let me be clear: if Russia continues in this direction, it will not just be a grave mistake, it will be an expensive mistake," he said during a press conference on Thursday.

Source: Al Jazeera and agencies