At least nine climbers are feared to have died in north Pakistan trying to scale the world's second-highest peak, K2, expedition organisers say.
There are fears more climbers could have died, or still be trapped, after an avalanche, but others did manage to reach safety.
A chunk from an ice pillar snapped away on Friday and broke ropes on a feature called the Bottleneck, reports said.
Only a few hundred people have climbed K2 and dozens have died in the attempt.
Many regard the mountain, at 8,611 metres (28,251 ft), as the world's most difficult peak to climb.
Expedition organisers only learned of the avalanche after a group of climbers arrived back at the mountain's base camp on Saturday evening.
Several search parties have since been despatched to rescue the remaining climbers, organisers said.
Fatality rate
There are reports some mountaineers may be stuck above the Bottleneck, unable to descend as the fixed ropes are broken.
Climbers call the area the Death Zone as lack of oxygen at that altitude can cause bodies to degenerate.
The weather is thought to be fair but the Pakistani military is still unsure whether it can launch a rescue attempt at such an altitude.
Renowned climber Reinhold Messner told the BBC the situation was "very critical" and those above the Bottleneck might have to try to climb down on the Chinese side to survive.
One Basque climber who reached safety, Alberto Zerain, told reporters mountaineers had hit trouble on their descent.
The mountaineers include Koreans, Pakistanis, Nepalis, a Dutchman and an Italian, reports say, but exact details remain unclear.
Buried on the mountain
A total of 22 climbers were thought to be high up on the mountain when the avalanche hit, says the BBC's Syed Shoaib Hasan in Islamabad.
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, our correspondent says.
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. Its leader, Dutchman Wilco van Rooijen, was reported safe, as was Italian climber Marco Confortola. But Frenchman Hugues d'Aubarede is 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.
