ISLAMABAD, Pakistan (AP) – At least nine climbers are feared dead and five others are missing on K-2, the world’s second-highest peak and long considered even more challenging than Mount Everest, officials said Sunday.

An avalanche struck a group of climbers on their way down after scaling the mountain in northern Pakistan, and at least six of them died. Three other mountaineers were reported to have died over the weekend in separate incidents.

In addition, five climbers from another expedition were missing, said Mohammed Akram, vice president of the Adventure Foundation of Pakistan, a nonprofit organization.

Akram said the six climbers died when they were hit by an “ice cornice” or snow crashing off an overhang on K-2 during the mountaineers’ climb down.

Nazir Sabir, whose Alpine Club of Pakistan organized a Serbian expedition on the mountain, said at least seven people were feared dead and three others were missing after they were struck by an ice avalanche about 1,100 feet below the peak on their way back.

It was not immediately possible to reconcile the differing tolls given by Akram and Sabir. A military-run helicopter service, which helps in rescuing climbers stuck up on high altitudes, had no information on any deaths on K-2.

In addition to the avalanche, Sabir said that a Pakistani climber and a Serbian climber fell to their deaths a day before the avalanche. A Korean climber also died over the weekend and Akram said the cause was undetermined.

K-2 is 28,250 feet tall – about 785 feet shorter than the world’s highest peak, Mount Everest – but climbers generally regard K-2 as the more difficult to summit.

Nestled in the northern regions of Pakistan, home to five of the world’s 14 tallest peaks, K-2 is so remote it is not visible from any inhabited place. It takes four or five days of hiking, or a 90-minute helicopter flight, to get to base camp.