Pakistan uses military helicopters widely in the rugged north-west region
Up to 26 Pakistani security personnel are feared dead after an army transport helicopter crashed in the tribal region of Orakzai, military officials say.
Maj Gen Athar Abbas told the BBC the helicopter crashed on the border of the Khyber and Orakzai tribal region.
The cause of the crash is unclear, although officials said the most likely explanation was a technical failure.
The crash comes as a suspected US drone strike in South Waziristan killed at least 10 militants, officials said.
The BBC's Mike Wooldridge in Islamabad says it is understood the MI-17 helicopter had been flying back to Peshawar from the Afghan border region when the pilot put out a Mayday alert.
The helicopter then came down "in a hostile area" where it was fired upon by militants, according to officials.
Troops were sent in and exchanged fire with the insurgents.
Military officials said that an investigation into the crash would be carried out.
But our correspondent says it is a serious blow for the Pakistani military as it prepares for the next phase of its offensive against Taliban militants in the north-west tribal belt along the Afghan border.
In the latest fighting, military jets are reported to have attacked suspected Taliban positions in South and North Waziristan.
Unnamed intelligence officials said the drone attack in South Waziristan had targeted a militant training facility.
The region - on the Afghan border - is controlled by Pakistan's most senior Taliban leader Baitullah Mehsud.