Ukraine’s military has launched an assault on the flashpoint rebel-held town of Slavyansk, sending in armoured vehicles and a helicopter, AFP journalists in the town report.

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

Shooting was heard as a helicopter flew overhead and pro-Russian gunmen in the town retreated to defensive positions.

A spokeswoman for the separatists, Stella Khorocheva, told AFP that all civilians had been ordered out of the insurgent-held town hall.

“The armed men handling the defence of the town hall will remain,” she said.

Kiev on Thursday claimed it had inflicted stinging defeats on pro-Kremlin rebels in Ukraine’s east, as US President Barack Obama accused Russia of not abiding by a deal to defuse the escalating crisis in the ex-Soviet country.

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 Moscow – which Kiev and Washington accuse of controlling the insurgency and which has tens of thousands of troops massed on the border – warned on Wednesday it could strike back if its interests in Ukraine were attacked.

Washington, meanwhile, has begun deploying 600 US troops to boost NATO’s defences in eastern European states bordering Ukraine.

Russian Foreign Minister Sergei Lavrov on Thursday accused the US and the European Union of trying to stage “an operation to unconstitutionally change the regime,” the Interfax news agency quoted him 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 he threatened further sanctions.

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.

Separatist sources in the east meanwhile confirmed they had lost the town hall in Mariupol, a port city on the Black Sea with a population of nearly 500,000. The separatists had held the town hall since April 13.

According to Interior Minister Arsen Avakov, “the town hall is liberated and can function normally”.

In Artemivsk, just north of the rebel-held hub of Donetsk, Ukraine’s defence ministry said in a statement that nearly 100 separatists “opened fire with automatic weapons, machine guns and used grenades” in an overnight attack on the military base where a soldier was wounded.

“The attackers were repelled and suffered significant losses,” acting president Oleksandr Turchynov said.

The Ukrainian government announced an offensive against the rebels following the discovery in a river near Slavyansk of a weighted down body of an abducted local politician who belonged to Turchynov’s party.

Russia has said it could respond as in 2008, when it invaded Georgia with tanks to support pro-Russian regions seeking independence.