BEIRUT — After rebel gains, the leader of the Syrian opposition made his first visit Sunday to areas near the embattled northern city of Aleppo as fighters trying to oust President Bashar Assad captured a police academy and a border crossing with Iraq.

Assad, meanwhile, lashed out at the West for helping his opponents in the civil war, delivering a blistering rebuke to Secretary of State John Kerry’s announcement that the United States will for the first time provide medical supplies and other nonlethal aid directly to the rebels in addition to $60 million in assistance to Syria’s political opposition.