KATHMANDU, Nepal - An avalanche sweeping down Mount Everest killed at least 12 Nepali guides Friday in what may be the deadliest single incident on the world's highest peak.

The avalanche struck a perilous passage on the main route to the summit as sherpas were preparing the way for climbers at the start of the season. Nepal's tourism ministry listed the 12 dead and four others missing, presumed buried in the snowslide.

Scottish filmmaker Ed Wardle put the death toll at 16 - including five from his own party - with more badly injured.

"One of the most horrific sites I ever saw on Everest was seeing the bodies being airlifted on long lines below the helicopters," he told Britain's Channel 4 News.

It was Everest's first major avalanche of this year's climbing season, when hundreds of foreign and Nepali climbers will attempt to reach its 29,029-foot peak.

The Himalayan Guides, a Nepali hiking group, said six of its sherpas had gone ahead of climbers they were accompanying, in order to fix ropes and crack snow and ice to carve out a route, when they were caught in the avalanche and died.

"They were very strong and skilled climbers. It is a natural disaster and no one could do anything about it," Ishwari Paudel, the owner of the company, said in Kathmandu. The other victims were working for other mountaineering parties.

More than 4,000 climbers have scaled Everest's summit since it was first climbed by Sir Edmund Hillary and Tenzing Norgay Sherpa in 1953.

The search for the missing was called off for the night and will resume Saturday, Lakpa Sherpa of the Himalayan Rescue Association told Reuters from the base camp.

"The atmosphere at base camp is now of shock and of grieving," Wardle told Channel 4, adding that "many of the expeditions here will pack up and go home. For this number of people to die at the very beginning of the season is completely unacceptable."

The avalanche, which hit around 6:30 a.m., left dozens of other climbers who had reached more advanced camps on the mountain unable to return to base camp, mountaineer and blogger Alan Arnette wrote from the scene.

The avalanche struck at the Khumbu icefall, a treacherous passage riddled with crevasses and columns of ice known as seracs, Adrian Ballinger of California-based Alpenglow Expeditions told Reuters. "In many ways it's always the most dangerous part of the mountain to climb, because the ice is constantly moving," he said.

A travel blog for Ed Marzec, 67, of California, attempting to become the oldest American to climb Everest, reported that he was among tourists preparing to set out when the avalanche happened. "We've just heard from Ed. He is safe and sound at base camp," the update said.