Security forces attacked the madrassa located north of Khar, the headquarters of Bajur tribal agency, at 5 am.

"There were casualties as between 70 to 80 people were present at the madrassa when the security forces conducted the operation," army spokesman Major General Shaukat Sultan said.

The exact number of casualties is not known.

"The attack was launched after confirmed information was received that the inmates were involved in terrorist training," Sultan said adding that the seminary was being observed for few days.

"Gunship helicopters were used and most of the targets were eliminated," he was quoted as saying.

It is the second major attack on Bajur in less than a year.

Locals said a local leader of a banned group 'Tanzeem Nifaz-e-Shariat-e-Muhammadi' owned the targeted madrassa.

In January, the US launched a missile strike on a village in the Bajur area in an effort to kill al Qaeda's deputy leader Ayman al-Zawahiri.

The strike killed 18 people but Pakistani officials said that al-Zawahiri was not among the dead.