BEIRUT — The battle for Syria’s second-largest airport intensified Saturday as government troops tried to reverse recent strategic gains the rebels have made in the northeast in their quest to topple President Bashar Assad.

Assad’s forces have been locked in a stalemate with rebels in Aleppo since July when the city, the largest in Syria, became a major battlefield in the two-year-old conflict the United Nations says has killed at least 70,000 people.