German Chancellor Angela Merkel has demanded an explanation from Russia's President Vladimir Putin over Russian troops’ incursion into south-east Ukraine, reports BBC.
Russian forces had crossed the border and were supporting separatist attacks, Ukraine said.
However, Russia has repeatedly denied arming or covertly supporting the rebels.
It suspected a Russian-directed counter-offensive was under way, according to the US State Department.
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.
Russia was not involved in the current offensive, Denis Pushilin, a rebel leader in Donetsk, told a news conference.
"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.
