An explosion has killed three children and injured at least 10 others, six seriously, at a religious seminary in the Pakistani city of Karachi, police sources have said.

Police sources said the attack at the Jamia Masjid Tahiriya madrassa on Monday was caused by a hand grenade thrown inside.

Investigators are unsure who detonated the device. An unnamed officer, speaking to the Reuters news agency, speculated that the Chinese-made grenade could have been brought in by a student.

"We desperately called various ambulance services so that the injured could get medical treatment," said witness, Naseebur Rehman.

"Meanwhile people shifted the injured in rickshaws and other vehicles to Abbasi Shaheed hospital." 

The madrassa is in the Frontier Colony area of Karachi's SITE industrial area.

Javed Odho, a police official in Karachi, said the mosque belongs to the country's majority Sunni sect and that there was so far no explanation as to why it had been targeted.

Odho said that the blast happened as students were in class, the AP news agency reported.

Al Jazeera's Asad Hashim, reporting from Islamabad, said the blast is the third attack in a week in Karachi, a city where targeted killings for political, religious and ethnic reasons have been common in recent years.

On April 24, police officer Shafiq Tanoli and three others were killed when an IED went off near his residence in the old fruit and vegetable market area of the city.

A day later, four people were killed and 30 injured in an attack outside a mosque in the Delhi Colony area of the city.

Source: Al Jazeera and agencies