ISLAMABAD — A suicide car bomber attacked a prison van while gunmen torched six NATO oil tankers in separate strikes Saturday that killed four Pakistani police officers and wounded 10 others, authorities said.

The army, meanwhile, kept up its pressure on the Pakistani Taliban in the tribal belt, killing 20 suspected fighters, while apparent U.S. missiles killed five alleged insurgents in a nearby northwest region, officials said.

The oil tankers were hit in Chakwal district — a rare, possibly unprecedented such assault in Punjab province. Militants and ordinary criminals frequently attack trucks that travel along supply routes used by NATO and U.S. forces in Afghanistan, but usually in the northwest Khyber-Pakhtoonkhwa or southwest Baluchistan provinces.

Suspected militants in two pickup trucks rode up to the gas station where the tankers were parked and opened fire before setting the vehicles aflame, police officer Aslam Tareen told The Associated Press. Four police officers who responded to the scene were killed, he said.

The drivers of the oil tankers said they were headed for NATO troops in Afghanistan, Tareen said. The militants fled. Chakwal is not far from the Punjab border with the northwest province.

On Saturday morning, a suicide car bomber targeted a prison van as it arrived at a jail in Timergarah to pick up prisoners to take to the nearby Swat Valley, senior police official Shakeel Khan said.

No prisoners were in the van at the time, but 10 police officials were wounded.