Russia pressed over Ukraine fighting

German Chancellor Angela Merkel has demanded an explanation from Russia's President Vladimir Putin amid reports that Russian troops have launched an incursion into south-east Ukraine, the BBC reported.

Ukraine said Russian forces had crossed the border and were supporting separatist attacks.

The US state department said it suspected a Russian-directed counter-offensive was under way.

Russia has repeatedly denied arming or covertly supporting the rebels.

The BBC's Barbara Plett Usher in Washington says the suspicion is that Moscow is opening a new front to divert Ukrainian forces from the besieged cities of Donetsk and Luhansk, where they have made significant advances against pro-Russian separatists.

However, Denis Pushilin, a rebel leader in Donetsk, told a news conference that Russia was not involved in the current offensive.

"If Russia entered into the war the counter-offensive would already be in Kiev. For now, we do without outside help," he said.

He added that the rebel forces were receiving more volunteers, some from as far away as Serbia.

Earlier on Wednesday, the mayor of the Ukrainian port of Novoazovsk on the Sea of Azov said rebel tanks and armoured vehicles had entered the town.

Rebels have been trying for weeks to break out of an area further north in the Donetsk region where they are almost encircled.

Analysts say the separatists could also be seeking a land link between Russia and Crimea, which also would give them control over the entire Sea of Azov.