(CNN) -- A bomb blast at a hospital in northwest Pakistan killed at least 23 people Tuesday, authorities said. Another 15 to 20 were injured.
The explosion occurred near the emergency entrance to the Dera Ismail Khan District Hospital, said Malik Naveed, inspector general of police for the North West Frontier Province, where the district is located.
The bomb went off while a crowd gathered outside the hospital to protest the shooting death of a Shi'ite Muslim leader, Naveed said.
No one immediately claimed responsibility for the explosion, but police think a suicide bomber may have been involved.
The North West Frontier Province, near the country's border with Afghanistan, is rife with Islamic extremists.
It has been the site of recent clashes between Pakistani security forces and militants. Watch what happened at hospital »
In late June, the military launched an offensive in the area -- the biggest push against extremists in the tribal region since the civilian government took power in March. Islamic militants vowed to retaliate against the operation.