Up to 26 Pakistani security personnel are feared dead after an army transport helicopter crashed in the tribal region of Orakzai, military officials say.

"The helicopter crashed on the border of the Khyber and Orakzai tribal region," Maj Gen Athar Abbas said.

"There were up to 26 troops on board, including officers," he told the BBC.

It is not clear how the crash took place, although officials said the most likely explanation was a technical failure.

The BBC's Mike Wooldridge in Islamabad says it is understood the MI-17 helicopter had been flying back to Peshawar from the northern tribal regions.

When the helicopter came down, it was fired upon by militants, officials said.

Troops were then sent in and exchanged fire with the insurgents.

The north-west of Pakistan is currently the scene of heavy fighting between the Pakistani army and Taliban militants.