Gunfire at Pakistan hospital treating bombing victims

Islamabad, Pakistan (CNN) -- Militants have entered a hospital in Quetta, Pakistan, where casualties of an earlier bus bombing were taken for treatment, police said Saturday.

Eight gunmen are exchanging fire with police at the Bolan medical complex, authorities said. Doctors, nurses and patients are trapped inside the hospital, which is surrounded by security forces.

Among the patients are more than 20 people wounded in a bus explosion that also killed at least 11 female university instructors, police said.

 "All the victims are women teachers and students," according to a police official, Mir Zubair Mehmood, in Quetta. The blast shattered the windows of offices and classrooms inside Sardar Bahadur Khan Women University.

The university bus was parked at the school, located on the outskirts of Quetta, the capital of Balochistan province.

There was no immediate claim of responsibility.