A bomb blast outside the emergency gate of a hospital in Pakistan's volatile northwest killed at least 12 people and wounded 15 others Tuesday, a police inspector said.

The attack occurred a day after Pervez Musharraf stepped down as Pakistan's president, adding to the political uncertainty in a country grappling with Islamist extremist threats.

It also occurred amid an ongoing military offensive in a nearby tribal region that has left hundreds dead and spurred promises of militant revenge.