A bomb blast at a religious school in the Pakistani city of Peshawar on Tuesday killed at least seven people and wounded more than 80, police and hospital officials said.

There was no immediate claim of responsibility for the bomb in the northwestern city, near the border with Afghanistan, which has long been plagued by Islamist violence.

There have so far been conflicting reports about the number of children among the dead or the injured.

"Unknown people planted explosives in a plastic bag," said a police officer who declined to be identified as he is not authorised to speak to media.

Initial reports stated at least four students were among the dead, though their ages are still unclear

Peshawar police chief Mohammad Ali Gandapur told Reuters the bomb contained up to 6 kg (13 lb) of explosives.

Though militant violence in the city, and in Pakistan in general, has fallen over recent years, there has been an increase in attacks on the security forces this year in Khyber Pakhtunkhwa province, of which Peshawar is capital.

Some of the attacks have been claimed by the Pakistani Taliban.

Parents and friends gathered at the school in tears in the wake of the explosion

Tariq Burki, director the city's Lady Reading Hospital, said earlier that four of the seven dead were children but later said four children were wounded and all the dead were adults.

Five of the 83 wounded were in critical condition, he said.

The school was seriously damaged in the blast

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.

The Madrassa's cleric was in the midst of delivering a sermon on Islam, it has been claimed

Others report that 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 madrassa.