A high-altitude avalanche on Mount Everest Friday morning killed at least 12 Sherpa guides and seriously wounded three in the single deadliest accident ever on the world’s highest mountain, Nepali officials said.

Four others are missing, said Madhu Sudan Burlakoti of the Tourism Ministry, adding that six people were injured in total, CNN reported.

A group of about 50 people, mostly Nepali Sherpas, were hit by the avalanche above 20,000 feet, said Tilak Ram Pandey of the ministry's mountaineering department.

The Sherpa guides had gone early in the morning to fix ropes for climbers when the avalanche hit just them below Camp 2 around 6:30 a.m., Tourism Ministry official Krishna Lamsal told the Associated Press from the base camp, where he is monitoring rescue efforts.

The avalanche took place just above base camp in the Khumbu Ice Fall. The area is nicknamed the "popcorn field" for its bulging chucks of ice and is just below Camp 2, Ang Tshering of the Nepal Mountaineering Association said. Camp 2 sits at an elevation of 6,400 meters (21,000 feet) on the 8,850-meter (29,035-foot) mountain.

The climbers were accounted for, Pandey said. "Rescue teams have gone ... to look for the missing."

Before Friday, the deadliest single-day toll occurred during a storm in 1996, when eight climbers disappeared.

Between May 15 and 30 is usually the best window for reaching the 29,028-foot peak.

Climbers and guides had been setting the ropes for the route, acclimating to the climate and preparing the camps along the route when the avalanche hit Friday, Gordon Janow of Alpine Ascents International in Seattle told CNN.

Climbers arrive in April to acclimate to the altitude before heading toward the summit of the world's highest mountain.

Sherpas, natives of the region, act as guides for the mostly foreign clients.

The spring climbing season is the busiest of the year. About 334 foreign climbers have been given permission to climb Everest over the next couple of months, with an estimated 400 Sherpas helping them, mountaineering official Dipendra Poudel said.