At least 12 local guides have been killed after an avalanche on the slopes of Mount Everest, Nepali officials say.
The avalanche struck around 06:45 local time (01:00 GMT) in an area known as the "popcorn field", just above Everest base camp at 5,800m (19,000ft).
A spokesman for Nepal's tourism ministry told the BBC some missing climbers had been rescued, but more are still missing.
It is thought to be the deadliest climbing tragedy on the mountain.
The Sherpa guides had climbed up the slope early in the morning to fix ropes for climbers and prepare the route for mountaineers when the avalanche hit, officials are quoted as saying by the Associated Press news agency.
The identities of the dead and missing have not been officially disclosed, tourism ministry spokesman, Mohan Krishna Sapkota, told the BBC.
He said a search and rescue operation was now under way and that three helicopters have been sent to the area.
Several injured climbers have been brought to the base camp.
Correspondents say the accident is a reminder of the risks Sherpa guides undertake in preparing the mountain for climbers. An equal number of guides ascend the slope to help foreign mountaineers, often preparing the way by fixing ropes and setting up camp.
This accident comes during the peak climbing months of April and May as hundreds of climbers converged at base camp in the hope of scaling the summit.
In 1996 eight climbers died during a storm and the disaster eventually formed the basis of the best-selling book Into Thin Air.
More than 3,000 people have scaled Mount Everest since it was first conquered by Edmund Hillary and Tenzing Norgay in 1953, but many have died in the attempt too.
Straddling Nepal and China, the world's highest mountain has an altitude of 8,848m (29,029ft).