ISLAMABAD - A PAKISTANI military helicopter crashed in the northwest of the country on Friday killing all 26 security personnel on board, officials said, ahead of a planned army offensive against a Taleban militant chief.

The helicopter crashed because of a technical fault about 20km from the city of Peshawar on the mountainous border of the Orakzai and Khyber ethnic Pashtun tribal regions, the official said.

Pakistani soldiers have been fighting Taleban militants in the Swat valley, northwest of Islamabad, for two months and have stepped up pressure on Pakistani Taleban leader Baitullah Mehsud, in his South Waziristan stronghold near the Afghan border.

'An MI-17 helicopter crashed due to technical fault, a military official who declined to be identified said, adding that 26 security personnel had been killed.

All those on board had been killed, said another military official.

Pakistan's civilian government has said it is determined to fight militancy and defeat Mehsud and his followers.

The army action followed an alarming expansion of Taleban influence and aggression in the northwest.

US officials have welcomed the offensive after earlier voicing fears about Pakistan's stability and the safety of its nuclear arsenal.

The military says it is nearing the end of the offensive in the former tourist valley of Swat, although soldiers are encountering pockets of fighters, and it has been preparing for an offensive against Mehsud.

The army has been carrying out air strikes by jet fighters to soften up targets while soldiers have been securing roads into Mehsud's area in the rugged mountains of South Waziristan.

Earlier on Friday, a US drone aircraft fired missiles into a Mehsud stronghold in the region, killing 10 militants, security officials said.

The United States, facing a growing Afghan insurgency, began stepping up drone attacks on militant strongholds in lawless enclaves on the Pakistani side of the border a year ago despite Pakistani complaints. -- REUTERS