The number killed or missing was unclear Sunday, with Spanish media reporting up to 11 people might have been swept away, citing a blog linked to an 18-member expedition.

The pyramid-shaped K2, which sits on the border between Pakistan and China, is considered by mountaineers to be by far the hardest of the 14 summits over 8000 metres to scale.

Weather patterns in the high-altitude Karakorum range where the mountain is located are also extremely volatile.

Basque daily Gara quoted Basque climber Alberto Zerain as saying he had reached the summit of K2 on Friday evening but a "drama" had hit several members of his expedition.

Swedish climber Fredrik Straeng told Swedish news agency TT 11 climbers died.

"I have carried down both living and dead people from the mountain. I panicked when a Pakistani high-altitude carrier fell straight onto my back with his entire weight.

"I was terrified that he would pull us all off the cliff and screamed to him to use his ice axe, but he lost his grip and plummeted off a 300-metre cliff," Straeng told TT.

Straeng said a large number of climbers decided to leave their camp at just over 7000 metres to try to reach the peak after the skies cleared following a long period of poor weather.

"We had a feeling this would not turn out well and decided to turn around. The accident could have been prevented. These mountains lure way too inexperienced and naive people," he said.

Gerard McDonnell, 37, an Alaska-based oil worker from Limerick in south-western Ireland who has climbed Everest, was given up for dead by an experienced mountaineering friend.

"At present it is believed that anyone who is classified as position unknown will not be coming back," Pat Falvey said.

Two Dutch colleagues who were on McDonnell's expedition were heading to base camp, said Michel Schuurman of Incipit Expeditie management, which organised logistics and sponsorship for the Dutch team.

"The lower they come to the ground, the closer they are to safety. But they are totally exhausted and not out of danger yet," Schuurman said.

Norwegian media are reporting that climber Rolf Bae, 33, died in the disaster, while his wife is reportedly trying to make her way down with two other Norwegians.

Separately, government officials said on Saturday that Serbian mountaineer Dren Mandic and another unidentified member of his expedition died after falling into a crevasse on K2 last month.

Deputy Commissioner of Skardu town Wazir Ishfaq said the expedition group, which began its climb in June, had informed authorities of the casualties on their return.

K2, known in the local language as Chogori or King of Mountains, has a ratio of climbers to deaths of 27 per cent, three times that of Everest.

Italian climbers Achille Compagnoni and Lino Lacedelli first scaled the mountain on July 31, 1954. Between that first ascent and 2007, there were 284 successful ascents and 66 fatalities.

Over the years, K2 has claimed the lives of some of the world's most accomplished mountaineers, including Britain's Nicholas Estcourt, Alan Rouse, Julie Tullis and Alison Hargreaves, American Rob Slater and France's Lilliane and Maurice Barrard.