Hamid Shakeel, the chief of police in Quetta, says the attackers opened fire on the officers Sunday during a patrol on the city’s outskirts. It was not immediately clear who was behind the attack.

But Quetta is the capital of Baluchistan province, where nationalist groups fighting for a greater share of the money derived from the region’s natural resources have for years waged a low-level insurgency.

Islamist militants have also been operating in and around Quetta.