try to grind down the ukrainian military with no regard for civilian casualties. where that leads, i think, is for an ugly next few weeks. >> now, ukraine's armed forces acknowledged russia's announcement on a proposed cease-fire but said, quote, it is difficult to trust the occupier. the events on the ground tuesday underscoring that suspicion. oren liebermann with our report. >> reporter: in northeast ukraine, not far from the russian border, the city of sumy was supposed to be safe, if only for a few hours. ukraine and russia agreed on a single evacuation corridor open for half of tuesday, but the agreement has not protected the city. the announcement came after ukrainian officials say a russian air strike killed 21 civilians, including 2 children overnight. russian strikes have destroyed homes in the city, flattening neighborhoods. western leaders have already accused russia of targeting pre-approved safe routes in ukraine. the city of mariupol in the south has been isolated by