Shooting was heard in the rebel-held flashpoint town of Slavyansk, where a roadblock manned by insurgents was in flames, according to a journalist. Seven army armoured vehicles were approaching the town, one militant said.
Ukrainian Special Forces retook control of the town hall in the southeastern port city of Mariupol and an army base in the eastern town of Artemivsk repelled an attack by heavily-armed rebels, Kiev's interior and defence ministries said.
They were the first military successes announced by Ukraine's Western-backed government since pro-Russian militants seized control of a string of towns in the country's southeast over the past several weeks.
But they also greatly raise the stakes in the Cold War-style crisis after Moscow - which Kiev and Washington accuse of controlling the insurgency and which has tens of thousands of troops massed on its border with Ukraine - warned on Wednesday it could strike back if its interests in Ukraine were attacked.
Washington has begun deploying 600 US troops to boost NATO's defences in eastern European states bordering Ukraine, with the first unit of 150 soldiers arriving in Poland on Wednesday, with the remained due to land in the Baltics in the coming days.
Russian Foreign Minister Sergei Lavrov on Thursday accused United States and European Union of trying to stage "an operation to unconstitutionally change the regime," the Interfax news agency quoted Lavrov as saying.
"They are trying to use Ukraine as a pawn in a geopolitical game," he said.
Hours earlier, US President Barack Obama, on a trip to Japan, said Russia was not abiding by last week's agreement to defuse the Ukrainian crisis and issued his own warning that Washington was ready to slap fresh sanctions on Moscow.
The Kremlin, Obama said, was not abiding "by the spirit or the letter of the agreement in Geneva" struck a week ago between Russia, Ukraine and the West that was meant to de-escalate the tensions in Ukraine.
"We continue to see malicious, armed men taking over buildings, harassing folks who are disagreeing with them, destabilising the region and we haven't seen Russia step out and discouraging it," he said.
Kiev, on the other hand, has promised amnesty to the separatists, protection of the Russian language and decentralisation of power, Obama said.


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