"Everything was going well to Camp Four and on summit attempt everything went wrong," Van Rooijen told The Associated Press by phone from a military hospital, where he was being treated for frostbitten toes.
K2, which lies near Pakistan's northern border with China, is regarded by mountaineers as more challenging to conquer than Mount Everest, the world's highest peak. K2 is steeper, rockier and more prone to sudden, severe weather.
Van Rooijen said several expeditions waited through July for good weather to scale K2 and decided to go for the summit when winds dropped on Friday. In all, about two dozen climbers made the ascent, officials said.
But Van Rooijen said advance climbers laid ropes in some of the wrong places on the 28,250-foot peak, including in a treacherous gully known as as "The Bottleneck."
"We were astonished. We had to move it. That took of course, many, many hours. Some turned back because they did not trust it anymore," said Van Rooijen, 40.
He said those who went on reached the summit just before nightfall. As the fastest climbers descended in darkness across The Bottleneck, about 1,148 feet below the summit, a huge serac, or column of ice, fell. Rooijen said a Norwegian climber and two Nepalese sherpas were swept away. His own team was split up in the darkness.
The Ministry of Tourism released a list of 11 climbers believed dead: three South Koreans, two Nepalis, two Pakistanis and mountaineers from France, Ireland, Serbia and Norway.
At least two fell on their way up the mountain, before the avalanche.
Van Rooijen said after the avalanche there was a "whiteout" on the mountain - meaning cloud had descended, making it virtually impossible to see the precipitous route down. But he pushed on as he was starting to suffer snow blindness.
On his descent, he said he passed three South Koreans. They declined his offer of help.
"There was a Korean guy hanging upside down. There was a second Korean guy who held him with a rope but he was also in shock and then a third guy was there also, and they were trying to survive but I had also to survive," he said.
It was not immediately clear if they were the same three Koreans who died. Two other Koreans made it back to the base camp, which lies at about 16,400 feet, an organizer of their expedition said.
The Italian climber, Marco Confortola, descended to 20,340 feet but bad weather forced officials to abort a helicopter rescue Monday, said Shahzad Qaiser, a top official at the tourism ministry. He was climbing down on foot, despite frostbite, assisted by a support team from a base camp.
"Up there it was hell. During the descent, beyond 8,000 meters (26,000 feet), due to the altitude and the exhaustion I even fell asleep in the snow and when I woke up I could not figure out where I was," the ANSA news agency quoted Confortola as telling his brother Luigi by satellite phone.
"My hands are fine, while my feet are black from frostbite. Anyway, I can walk and I want to descend to the base camp." Agostino Da Polenza of Everest-K2-CNR, an Italy-based high-altitude scientific research group, also spoke to Confortola on Monday.
"I never gave up in my life, I am surely not going to give up now," Da Polenza quoted the climber as saying on his group's Web site.
Another attempt was planned for Tuesday, Qaiser said. The Irish climber, 37-year-old Gerard McDonnell, on Friday became the first person from his country to reach the summit of K2. He is believed to have died on the way down.
Pat Falvey, a family friend, said they "are holding up well and are very proud of Ger's achievement and are still in total shock in relation to the fact that he may not be coming back."
Before his death, 61-year-old Frenchman Hugues d'Aubarede gave an account of the climb - with freezing temperatures, bad weather and beautiful vistas - via a blog.
On the eve of his death, his last message from the foot of The Bottleneck was: "I would love it if everyone could contemplate this ocean of mountains and glaciers. They put me through the wringer, but it's so beautiful. The night will be long but beautiful."
The reported toll was the highest from a single incident on K2 since at least 1995, when seven climbers perished after being caught in a fierce storm.
About 280 people have summited K2 since 1954, when it was first conquered by Italians Achille Compagnoni and Lino Lacedell. Dozens of deaths have been recorded since 1939, most of them occurring during the descent.
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Associated Press writers Sadaqat Jan in Islamabad, Lily Hindy in New York and Kwang-Tae Kim in Seoul, South Korea, contributed to this report.
