Thirteen people died on Mount Everest early Friday after an avalanche swept down the route used to climb the world’s highest peak, NBC reported.

All of the dead and injured were believed to be Nepalese Sherpas, who were working to prepare the route with equipment and ropes ahead of the main climbing season which begins later this month, The Economic Times reported. The ethnic Sherpas work as guides for foreigners who come to the mountain.

About 100 Sherpa guides and climbers are thought to be trapped above the avalanche site, according to The New Indian Express.

The avalanche hit relatively low down on the mountain between base camp and camp 1, according to Reuters.

Soon after the avalanche hit at about 6:30 a.m., first responders and fellow climbers geared up to help. A helicopter was also dispatched from Kathmandu, AP reported.

”Rescuers have already retrieved four bodies and they are now trying to pull out two more bodies that are buried under snow,” Nepal Mountaineering Association president Ang Tshering Sherpa told Sydney Morning Herald.

The avalanche is the deadliest in eight years, according to Reuters.

The worst recorded disaster on Everest happened May 11, 1996, when eight climbers died after being caught in a blizzard close to the summit.

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