Pro-Russian rebels in Ukraine have entered the key port of Novoazovsk on the Sea of Azov, opening a new front in their fighting with government forces.

According to reports from journalists and military on the ground Novoazovsk has been captured by the rebels who are now believed to be advancing towards the port of Mariupol, the BBC said.

In Washington, the BBC said, 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."

According to some analysts, the pro-Russian separatists could be seeking to establish a land corridor between Russia and the Crimea, which also would give them control over the entire Sea of Azov.

Ukrainian authorities said Russian regular army units have crossed the border on Wednesday in support of rebel attacks. Ukraine, which seems to have lost control of a section of the border and believes Russian arms and military personnel are crossing unimpeded to aid the separatists. Moscow has repeatedly denied aiding the rebels.

German Chancellor Angela Merkel has phoned Russia's President Vladimir Putin demanding an explanation and emphasising Russia's major responsibility for de-escalation of the Ukraine crisis.

"The latest reports of the presence of Russian soldiers on Ukrainian territory must be explained," Merkel’s spokesman Steffen Seibert said.

The Kremlin has confirmed the phone call without giving details.

In Washington, state department spokeswoman Jen Psaki said the US suspected a Russian-directed counter-offensive was likely underway in Donetsk and Luhansk regions.

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