Eleven climbers have died in north Pakistan trying to scale the world's second-highest peak, K2, reports say.
An eyewitness says 25 climbers reached the summit on Friday, but nine were stranded and froze to death after an avalanche swept away their fixed ropes.
In the deadliest day in K2's history, another climber fell to his death and a porter died recovering a body.
The avalanche happened when a chunk from an ice pillar snapped away on a feature called the Bottleneck.
Swedish climber Fredrick Streng, who had decided not to attempt the summit, gave the BBC details of the events.
He said several expeditions, with a total of 25 climbers, had taken advantage of Friday's fine weather to reach the summit of K2.
But he said they returned late, and were tired when the avalanche struck out the fixed lines.
"There were still nine climbers above these ropes, which meant they had to spend the whole night out in the open," he said.
"The next morning they were still there, not moving. And people during the night had been hit by the avalanche. One had died, one had fallen at an earlier stage.
"At the end of Saturday, 11 people were confirmed dead."
Only a few hundred people have climbed K2 and dozens have died in the attempt.
Many regard the mountain, at 8,611m (28,251ft), as the world's most difficult peak to climb.
The Death ZoneSee route to climbing K2
The mountaineers included Koreans, Pakistanis, Nepalis, a Dutchman and an Italian, reports say, but exact details remain unclear.
Several search parties have since been despatched to rescue the climbers who are still missing, organisers said.
Climbers call the area the Death Zone as lack of oxygen at that altitude can cause bodies to degenerate.
Reports from the mountain's base camp say that two separate parties of Serbian and Norwegian climbers have been able to make it back and that a Serbian and a Norwegian had died on the slopes.
The Serbians say they buried their team member as it was impossible to bring his body back. The Norwegians say their companion was lost in the avalanche.
One of the climbers reported missing is Gerard McDonnell, 37, from County Limerick in Ireland, the first Irish person to reach the mountain's summit.
He was on the Norit K2 expedition. The Dutch leader of the expedition, and an Italian climber, were reported to be safe, but a French mountaineer was missing.
The fatality rate for those who reach the summit at 27% is about three times higher than that for Mount Everest.
One of the worst single-day death tolls was on Everest on 11 May 1996, when eight people died in summit attempts.
Six people fell to their deaths or disappeared during a storm on K2 on 13 August 1995.
The summit of K2 was first reached by two Italians, Lino Lacedelli and Achille Compagnoni, on 31 July 1954.