A powerful bomb blast ripped through an Islamic seminary on the outskirts of the northwest Pakistani city of Peshawar on Tuesday morning, killing at least seven children and wounding 70 others, police and a hospital spokesman said.

The bombing happened while a cleric was delivering a lecture about the teachings of Islam at the main hall of the Jamia Zubairia madrassa, said police officer Waqar Azim. He said initial investigations suggest the bomb went off minutes after someone left a bag at the madrasa.

Several of the wounded were in critical condition, and hospital authorities feared the death toll could climb further.

The madrassa is largely for adult students and many were studying when the blast happened, said city resident Abdul Rahim, whose said his 27-year old cousin was among the wounded.

"He told us they were attending a class when the blast took place," Rahim said.

Peshawar is the provincial capital of Pakistan’s Khyber Pakhtunkhwa province bordering Afghanistan. The province has been the scene of such militant attacks in recent years, but sectarian violence has also killed or wounded people at mosques or seminaries across Pakistan.

The latest attack comes two days after a bombing in the southwestern city of Quetta killed three people.

No group claimed immediate responsibility for the attack.

Peshawar was once the epicentre of militant violence in Pakistan, with jihadists targeting security forces and public spaces in the city near the north-western frontier with Afghanistan.