(CNN) -- Eleven climbers died on Pakistan's K2 mountain after an ice avalanche knocked down a fixed rope climbers were using to reach the summit, a mountaineer at their base camp said Sunday.
Pakistan's K2 is the second highest peak in the world.
Among the dead was a sherpa who had gone up K2 -- the world's second tallest mountain -- to help in rescue efforts, said mountaineer Fredrik Strang, who also assisted in the rescue attempts.
The deaths happened after 17 climbers, in different expeditions from around the world, had come together to make it to K2's peak on Friday, said Pat Falvey, a climber in Ireland who was in touch with some of the 17.
As the 17 were descending early Saturday, a "moving river of ice broke loose ... like an iceberg breaking loose from a glacier," knocking down the fixed rope the group had been using to move from higher reaches to a camp at a lower altitude, Falvey said.
The rope's collapse caused three climbers to fall to their deaths, said Falvey, who was posting online updates for one of the expeditions. iReport.com: Are you a mountain climber? Ever attempt K2?
Two climbers decided to go on and managed to return to base camp, but the rest decided to wait and hope rescuers could reach them, Falvey said.
The avalanche had created "icy, dangerous conditions" on the slope, Falvey said. As time went on and rescuers didn't come, the remaining climbers decided to continue their descent, but some of them fell to their deaths in the mountain's "bottleneck" area, Falvey said.
The rest eventually were helped down by rescuers, Falvey and Strang said. Falvey and Strang said the survivors told them what had happened but were not immediately in condition to speak publicly. See a map of K2 mountain »
Among the killed climbers were Dutch, Irish, Italian, French, Norwegian, Korean, and Nepalese citizens, Falvey said.
One of the killed was Irish climber Gerard McDonnell, Falvey said.
The bodies of the 11 may never be recovered, but rescuers and the mountaineers who made it down are certain the 11 are dead, Strang said.
The site of the accidents, about 5 miles up the mountain, is what climbers call the "Dead Zone" because the body would never recover if stuck in such freezing conditions with so little oxygen, Falvey said.
Strang said Sunday the death toll was not expected to rise, because no one else was believed to be missing.
Strang said the deaths could have been avoided. Too many people were climbing together "at a very slow speed" and should have begun their descent sooner, he said.
"Coming down at dawn, in the dark, with little oxygen is very, very dangerous," he said.
Though K2 is the world's second-highest mountain, many climbers consider it more technically challenging than Mount Everest, the world's highest peak.
Statistics compiled by AdventureStats.com suggest this was the deadliest incident at K2. The site says 66 people -- not counting those in the latest incident -- have died on the mountain since 1939.
A chart compiled by viewfinderpanoramas.org lists 284 climbers as having ever reached the summit of K2.