At least 11 students were killed and around 20 wounded when a bomb planted in a women's university bus exploded in troubled southwest Pakistan on Saturday, officials said.

"It was an improvised explosive device placed in the women university bus" in Quetta, the main city in the insurgency-hit Baluchistan province, city police chief Zubair Mahmood told AFP.

"The dead were all women students," he said.

The bomb exploded after the female students had boarded and the bus was leaving the university.

"We are investigating if it was a remote-controlled bomb," the police official said.

Another police official, Fayyaz Sumbal added the bus caught fire after the explosion and many students were critically wounded.