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, reports GHN based on BBC.
Ukraine said Russian forces had crossed the border and were supporting rebel attacks. The rebels have opened a new front near the port of Mariupol.
The US said it suspected a Russian-directed counter-offensive was ongoing.
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.
Reports from journalists and military on the ground suggest that the town of Novoazovsk on the Sea of Azov has been captured by the rebels.
They are now thought to be heading towards the port of Mariupol.
The port has until now been peaceful and cut off from rebel positions.
Rebels have been trying for weeks to break out of an area further north in the Donetsk region where they are almost encircled.