Fierce fighting raged here on Thursday between the Ukrainian military and pro-Russian separatist rebels, with hours of intense shelling casting grave doubts on the prospects of a unilateral cease-fire that was promised just a day earlier by the new Ukrainian president, Petro O. Poroshenko. As government forces intensified their drive to crush the separatist insurrection that has plunged Ukraine into conflict, explosions rocked villages between Seversk and Krasny Liman, a railroad hub north of the regional capital of Donetsk that could provide a lifeline to the blockaded rebel stronghold of Slavyansk. In another foreboding development, the NATO secretary general, Anders Fogh Rasmussen, said Thursday that Russia had redeployed several thousand troops along the Ukrainian border. A month ago, President Vladimir V. Putin had ordered soldiers withdrawn from the border area in what appeared to be an effort to defuse Ukraine’s crisis, along with statements that Russia […]