PESHAWAR, Pakistan (AP) — A Pakistani military helicopter crashed in a mountainous region in the country's restive northwest on Thursday, killing at least 11 people on board, police said.

The helicopter, which serves as a medical assistance aircraft, was on the way to the northern town of Gilgit when it went down near the district of Mansehra in the Khyber Pakhtunkhwa province, said local police official Fida Khan.

"Rescuers have so far retrieved 11 bodies from the wreckage," he said, adding that the cause of the crash was unclear and that an investigation was underway.

However, a senior army officer told The Associated Press that the helicopter was carrying 12 people— including military doctors, paramedics and crew members — and was flying to Gilgit to evacuate a critically ill soldier when it went down, apparently because of bad weather.

The officer said the helicopter had taken off from Rawalpindi, a garrison city near the capital, Islamabad. It crashed near Mansehra, about 200 kilometers (120 miles) northeast of the city of Peshawar. The officer, who spoke on condition of anonymity because he wasn't authorized to speak to reporters, had no other details and said the army had dispatched troops to the site of the crash.

The crash came months after another army helicopter crashed in the northern town of Naltar, killing four foreigners — ambassadors to Islamabad from the Philippines and Norway, as well as the wives of the ambassadors from Malaysia and Indonesia. Three crew members from the army were also killed at the time.

Pakistan's army has been fighting local and foreign militants in the country's northwestern tribal regions bordering Afghanistan for nearly a decade. The military launched a major offensive in the North Waziristan tribal region last year and claims it has cleared 90 percent of the region of militants.

North Waziristan was considered to be the headquarters of the Pakistani Taliban until June 2014, when the army launched the long-awaited operation there.

Associated Press writer Munir Ahmed contributed to this report from Islamabad.