Officials in northwest Pakistan say militants have attacked a police checkpoint, killing three police officers and wounding another.

Authorities say the insurgents struck before dawn Sunday in Shangla, a district of the troubled Khyber Pakhtunkhwa province, near the Swat valley.

Taliban militants took control of the scenic Swat valley in 2007. But the army launched a major offensive there two years ago and pushed the militants out of the area. Hundreds of insurgents were killed in the operation, which displaced an estimated 2 million people.

Pakistan's northwestern tribal region is considered a stronghold of al-Qaida and the Taliban.