Search operation on Mount Everest

A big search operation has started on Mount Everest after an avalanche struck on Friday.

Thirteen local guides known as Sherpas were killed and three guides are still missing.

The guides were preparing the route for the climbing season - a risky job many Sherpas do to make sure the climb is safe for others.

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Sherpas are local people who are highly skilled climbers. They are paid to help people from around the world climb Mount Everest, the highest mountain in the world.

They know the mountain and its terrain very well. Some Sherpas plot safe routes for climbing, fix ropes and act as mountain guides to ensure climbers' safety.

Some Sherpas are on hand to provide medical attention in an emergency and others cook food needed for the trip or set up camp for the mountaineers.

'Sherpa' refers to the Tibetan tribe of people who live in the mountainous region of Nepal, near Everest.