Ukraine has rejected any truce with pro-Russian separatists until the insurgents completely disarm, amid a call by Western leaders for Russia to intervene to calm tensions.
In a statement on July 8, Ukrainian Defense Minister Valeriy Heletey said "negotiations are possible only after the rebels lay down their arms once and for all."
Ukrainian military successes in recent days have forced most of the rebels to retreat to Donetsk and Luhansk.
The rebels have vowed to regroup in Donetsk and Luhansk after government forces retook Slovyansk, a major rebel stronghold.
Ukrainian President Petro Poroshenko has ordered troops to blockade the cities and cut rebels off from any further arms supplies.
Civilians would be allowed to leave, however.
The separatists said attempts at a military blockade would destroy any hopes for a political solution.
On July 7, reports said three bridges on roads leading into the separatist stronghold of Donetsk were destroyed by explosions, allegedly by separatists.
However, the rebels said the bridges -- located in the villages of Novobakhmutivka, Zakitne, and Seleznevka -- had been blown up by pro-Kyiv "saboteurs."
Pavel Gubarev, the self-described governor of the self-declared Donetsk People's Republic, has threatened a "real partisan war around the whole perimeter of Donetsk."
Fighting between Ukrainian forces and the separatists have left more than 400 people dead and thousands homeless since the uprising began in April.
Ukraine's government ended a unilateral 10-day cease-fire last week and has since stepped up its fight against the rebels.
Meanwhile, in a telephone call on July 7, the presidents of France and the United States urged their Russian counterpart, Vladimir Putin, to pressure pro-Moscow rebels in eastern Ukraine to hold talks with Kyiv.
The French presidency said in a statement that Francois Hollande and Barack Obama talked on the phone for 45 minutes with Putin.
It said they "called on President Putin to pressure the separatists so that they agree to a dialogue with the Ukrainian authorities and to reinforce control over the Russian-Ukrainian border."
Obama and Hollande also said that "a durable solution to the crisis in Ukraine can only be a political one."
In a separate statement, the White House said Obama and Hollande decided that Europe should impose further costs on Russia if Moscow does not immediately take steps to de-escalate tensions in eastern Ukraine.
The White House said they agreed that Russia should stop "destabilizing activities" such as allowing and facilitating the transit of weapons and fighters across the border and should cease its own military buildup near the Ukrainian border and its ongoing support for separatists.
With reporting by AP, AFP, and Reuters