At least seven people have died and more than 50 injured after an explosion during a class at a religious school in Pakistan, police have said.

Hospital officials say those killed in the attack, which took place in the northern city of Peshawar, were aged between 20 and 30.

Dozens of others were injured, including four under the age of 13.

No group has yet claimed responsibility. An investigation has been launched.

Earlier reports suggested four of the dead were children, but this has since been retracted.

The city of Peshawar, close to the Afghan border, has seen some of the worst of the violence during the Taliban insurgency in recent years.

Six years ago, gunmen stormed a military school in the city leaving more than 150 dead, including many children.

However, the number of attacks has greatly reduced in recent years.

What happened?

The blast took place at about 08:30 local time (03:30 GMT), police told the BBC.

About 60 people are understood to have been in the class at the religious school, known as a madrassa.

An eyewitness has told the police he saw a man enter the building with a bag of explosives shortly before the blast.

Images from inside the room show extensive damage.

Assistant Inspector General of Police (AIG) Shafqat Malik told Pakistan's Express Tribune newspaper that 5kg (11lbs) of explosives were used in the attack.

Hospital officials told Reuters news agency that they had received dozens of injured, many with burns.

The AFP news agency has quoted police as saying that that two teachers were among the injured.

Pakistan's Prime Minister Imran Khan has expressed "regret over the loss of tragic lives", strongly condemning the blast.