MUMBAI, India -- A predawn avalanche on Mount Everest killed at least 12 Nepalese guides in the single deadliest accident on the world's tallest peak, officials said Friday.

The guides were preparing ropes and ferrying supplies for commercial climbers southwest of the summit when the avalanche occurred at an elevation of about 20,000 feet, Nepalese tourism officials said.

About two dozen guides, many of them members of the ethnic Sherpa community, were swept away, triggering a major search-and-rescue operation involving helicopters and Nepalese military forces.

Twelve bodies were recovered and eight guides were found injured by rescuers picking through the thick ice and snow.

Several more guides were missing, the local Himalayan Times newspaper reported.

The avalanche took place along a route used to ascend Everest, in an area just below Camp 2 known as the "popcorn field" because of its jagged chunks of ice. About 100 guides were reportedly trapped above the site and were cut off from returning to camp.

An Australian climber who witnessed the avalanche, Gavin Turner, said it "came out of nowhere" and he was immediately surrounded in a cloud of snow and dust. Then he heard a large thud, he told ABC News.

"Without warning, a large chunk of ice broke loose," said Turner. "There were a few seconds of panic where I thought this is going to collect us."

The guides were preparing a route ahead of the start of the climbing season on Everest, which attracts the world's foremost mountaineers as well as wealthy tourists who fork over upward of $30,000 for a chance at the summit.

The guides are the engine of the industry, porting heavy packs and supplies for less experienced climbers and even veterans challenged by Everest's forbidding conditions.

The previous deadliest avalanche on Everest occurred in 1996, when eight climbers were killed in a snowstorm that was recounted in Jon Krakauer's book "Into Thin Air."