Ukraine neared a final showdown with pro-Russia rebels, after Kiev forced insurgents to retreat to the last major cities they control and Moscow showed no signs of intervening to help them.

On Sunday, Ukraine said it plans to lay siege to Donetsk, a regional capital of one million residents that is the political and economic center of eastern Ukraine, and pursue rebels who fled there from Slovyansk, which had been the base of rebel military resistance in the region until government forces recaptured it over the weekend.

Rebel forces, meanwhile, also appeared to be readying themselves for a fight for Donetsk, the proclaimed capital of their breakaway republic.

A fight for the city is fraught with risk for both Moscow and Kiev, which for now has gained the upper hand against rebels, but done so without the open intervention of Russia. As rebels are routed by Ukraine's military, pressure has been growing inside Russia for President Vladimir Putin to order such an intervention.

"I'm very disappointed," said Fedor Berezin, rebel deputy defense minister, of Moscow's lack of action. "That means it will be a long and bloody war until we all die valiantly on the barricades."