Top Taliban commanders were gathered inside the madrassa at the time of the attack in the northeastern province of Kunduz, the security source told AFP.

He said an unknown number of civilians were among the casualties that also included senior Taliban commanders who were "planning for the next spring operations".

"Several dead and at least 15 wounded", including children, were taken to the regional hospital in the provincial capital Kunduz, Naim Mangal, a doctor, told AFP.

Relatives of the wounded told an AFP photographer at the hospital that the attack happened during a graduation ceremony for the madrassa students.

A defence ministry spokesman confirmed an airstrike but described the location as a kind of Taliban "training centre" and denied civilians were among the casualties.

"Twenty Taliban, including the commander of their Red Unit in the district, and also a key member of the Quetta Shura were killed," Mohammad Radmanish said, referring to the insurgent group's elite unit.

The same number were wounded, he added.