Ukrainian Prime Minister Arseniy Yatsenyuk says his country is in the midst of an undeclared war, in a televised address Thursday.
"This is the first time in 69 years it has happened in conditions of war, even undeclared, but real war, that the Russian regime is waging against Ukraine," Yatsenyuk said.
"The enemy has crossed the border of Ukraine for the first time since 1941, it has breached its integrity, annexed part of its territory, sent saboteurs and terrorist groups here," he said.
The pro-Russia rebels say a referendum planned for Sunday will decide if eastern regions will be ensured more autonomy within Ukraine.
But they have left open the possibility of using it to seek independence or annexation by Russia.
On Wednesday, Putin declared Russian troops had pulled back from the Ukrainian border and had called for a delay of Sunday's vote.
Putin also called Ukraine's interim government's plan to hold a presidential election on May 25 a "step in the right direction."
It is not clear if referendum organizers will follow Putin's request to delay Sunday's vote. But recent polls show most Ukrainians want their country to remain a single, unified state, even in the largely Russian-speaking east.
In a nationwide poll, the Pew Research Center found 77 percent of Ukrainian citizens want their country to maintain its current borders. And in the east, as many 70 percent feel the same.