Pakistani soldiers gather beside an army helicopter at a military hospital where victims of a helicopter crash were brought for treatment in Gilgit on Friday.

AFP/Islamabad

A Pakistani military helicopter crashed on Friday, killing six people including the Norwegian and Philippine envoys, with the Taliban claiming they downed it as part of a plot to kill Prime Minister Nawaz Sharif.

The Pakistani Taliban's spokesman said the group had struck the aircraft with a ground-to-air missile hoping to assassinate Sharif - who officials said had been travelling to the same region in a plane.

"A special group of Tehreek-e-Taliban Pakistan had prepared a special plan to target Nawaz Sharif during his visit but he survived because he was travelling in another helicopter," militant spokesman Muhammad Khorasani said.

It was not immediately possible to verify the Taliban claim and the northern region where the chopper came down, Gilgit-Baltistan, is not known as a stronghold of the militant organisation.

The ministry of defence said it was investigating what caused the crash, in which the helicopter fell onto a school and set the building ablaze. Officials clarified the school was shut at the time.

The helicopter was one of three carrying a delegation of ambassadors to inspect projects on a three-day trip to Gilgit-Baltistan where they were set to meet with Sharif.  

A statement by Sharif's office had said he was on a plane - not helicopter - en route to the Gilgit area at the time of the incident, but turned back to Islamabad after news of the crash broke.

He was set to inaugurate a chair-lift at a ski resort, one of the region's top administrative officials told AFP.

Leif H. Larsen, the Norwegian envoy, and Domingo D. Lucenario Jr of the Philippines were killed along with the wives of the Malaysian and Indonesian ambassadors, as well as the helicopter's two pilots, according to official tweets by the army.

Sharif "expressed deep grief and sorrow" and announced a day of mourning, according to his office.

Polish ambassador Andrzej Ananiczolish and Dutch ambassador Marcel de Vink were also injured, the army said.

The Dutch Ministry of Foreign Affairs confirmed Vink's injury.

It was Pakistan's worst air crash since 2012 when a Boeing 737 passenger plane went down in Islamabad, killing 130 people.

In 1988, a plane crash killed Pakistan's then military-ruler General Zia-ul-Haq as well as the US ambassador at the time, Arnold Raphel.

'Loud explosion'

A senior local administration official said the crash set the school building on fire, but that no children were in class at the time.

"The school, built by Pakistan Air Force for the children of the area, was closed as part of a security plan for the prime minister's visit," he said. Earlier, an official had said that the school was open.

Sher Ahmed, a local resident who was near the site of the crash, confirmed that the area had been under heavy security in preparation for the visit since Thursday.

"I was in my garden with my family watching the helicopters arriving when we heard a loud explosion and then the school building was in flames," he said.

According to a list of passengers obtained by AFP, the ambassadors of Indonesia, Lebanon, Malaysia, the Netherlands, Romania, Norway, South Africa, the Philippines and Poland were scheduled to fly on the helicopter.

"It was a diplomatic trip with members of 37 countries in total," said a passenger in one of the helicopters, who requested anonymity, concurring that the school had caught fire after the crash.

The passenger added that the air convoy was supposed to have included four helicopters but the number was later reduced to three.

The injured were being airlifted to a military hospital in Gilgit, the region's administrative capital, some 50 kilometres to the southwest, added another senior local police official.

In the city of Gilgit, a hospital official said the injured were being carried on stretchers to the emergency ward of the Combined Military Hospital.

Known for its spectacular mountain ranges, Gilgit-Baltistan is a strategically important autonomous region that borders China, Afghanistan and Indian-held Kashmir.