DAMASCUS, Syria —Sporadic violence persisted in Aleppo on Wednesday as U.S. officials announced an agreement had been reached with Russia to extend Syria’s fragile cease-fire to the deeply contested northern city. The Syrian military said the truce would last only 48 hours. Restoration of a partial truce would bring relief to residents on both sides of Syria’s largest city after two weeks of relentless violence that has killed nearly 300 people, destroyed hospitals and brought it to the brink of humanitarian disaster. It was not immediately clear whether the new effort will be observed or for how long.