Militants in Pakistan overnight ambushed security forces who were responding to an earlier attack on aid trucks in the country's troubled northwest, leading to a shootout in which four troops were killed, officials said Tuesday.

The ambush happened hours after authorities dispatched reinforcements to respond to Monday's attack on a convoy of aid trucks in which a driver and security official were killed in Kurram, a district in the restive Khyber Pakhtunkhwa province.