Pro-Russian separatists retreating ahead of a Ukrainian military offensive in eastern Ukraine dug in Monday in the city of Donetsk, building barricades and erecting checkpoints along main roads.
As Ukrainian forces pushed south from Slovyansk toward Donetsk, three bridges linking the two cities were destroyed in explosions.
It was not clear whether rebels triggered the blasts to slow the Ukrainian offensive or if government troops targeted the structures in a push to keep supplies and arms from reaching rebels in the city.
Since Ukraine seized control of Slovyansk Saturday, Ukrainian defense officials have said they plan a full blockade of Donetsk and the nearby rebel-controlled city of Luhansk. Separatists in both cities have declared independence from the Kyiv government and have sought to establish their own "people's republics."
The Kyiv government push to regain control of Ukraine's largely Russian-speaking east gained momentum last week, after President Petro Poroshenko refused to extend a unilateral cease-fire and ordered troops to advance.
In other developments, Russia's Itar-Tass news agency says Energy Minister Alexander Novak plans to meet next week with his European Union counterpart for natural gas supply talks.
Those talks would follow weeks of debate on how to resolve a dispute over how much Moscow charges Ukraine for gas supplies. The payment stalemate resulted in Russia cutting off gas supplies to its energy-dependent neighbor last month. A series of three-way EU-brokered talks have since failed to resolve the crisis.
Russia and Ukraine have been embroiled in the energy standoff since April, when Moscow nearly doubled the price of gas to $480 per 1,000 cubic meters and Kyiv refused to pay the higher charges.