Gunmen have taken over parts of a hospital in southwestern Pakistan Saturday after two bomb blasts in the city, including one that went off inside the hospital's emergency room, killed at least 12 people according to officials.

Armed men captured different sections of the hospital complex and were positioned on rooftops, said the head of police operations in Quetta, Fayaz Sumbal.

Frontier Corps troops, and police commandos have been called in, and security officials were trying to clear the hospital as quickly as possible, Sumbal said.

An Associated Press reporter on the scene could hear intermittent gunfire as troops took up positions around the building.

The violence in Baluchistan, a vast province in southwestern Pakistan, started when a blast ripped through a bus carrying female university students.

At least 11 people died in the incident, and another 19 were wounded, said police chief, Mir Zubair Mahmood.

As family, friends and rescuers crowded the emergency room where the dead and wounded were taken, another blast went off in the corridor.

A top government official was killed in the blast and two other people wounded, said Sumbal.

It was not immediately clear if the two events were related and there was no immediate claim of responsibility but militants often stage such coordinated attacks in order to target rescuers and others as they flood to the hospital.

The militants appeared to use the blast in the hospital as cover to disperse around the building.

Footage on Pakistani television of the still unfolding crisis showed people fleeing from the hospital after the explosion and hiding behind ambulances in the parking lot.

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Associated Press writer Zarar Khan in Islamabad contributed to this report.