An avalanche swept down a slope of Mount Everest on Friday, killing nine Nepali mountaineering guides at the beginning of the main climbing season, a tourism ministry official said.

The avalanche hit the most popular route to the mountain’s peak and three Nepali guides were injured and some people may be missing, Tilak Ram Pandey, an official at the ministry’s mountaineering department, told Reuters.

It was the first major avalanche on Mount Everest this climbing season, when hundreds of foreign and Nepali climbers flock to the mountain to attempt to climb its 8,850 metre (29,035 feet) peak.