An American climber died during his descent from the summit of Mount Everest on Monday, a Nepalese official said, bringing the number of dead or missing mountaineers on the world’s highest mountain to at least nine on the Nepali side during the current climbing season.

Christopher John Kulish, 61, scaled the 8,850 metre peak from the normal Southeast Ridge route in the morning but died suddenly at South Col after descending from the summit, Mira Acharya, a Nepal tourism department official said.

The authorities did not say where he was from in the United States. The cause of his death was unclear.

Most of the deaths on Everest this year have been attributed to exhaustion and tiredness, exacerbated because a crowded route to and from the summit has led to delays. The short climbing season ends this month.

The route, also called the South Col route, was pioneered by New Zealander Sir Edmund Hillary and Nepalese Sherpa Tenzing Norgay in 1953.

A total of 41 teams made up of 378 climbers are permitted to scale Everest during the spring climbing season. Nepalese guides called Sherpas help them get to the summit.

About half a dozen climbers died this past week, most of them while descending from the summit during only a few windows of good weather each May. Most are believed to have suffered from altitude sickness, which is caused by low amounts of oxygen at high elevation and can cause headaches, vomiting, shortness of breath and mental confusion.

Among the latest fatalities was British climber Robin Haynes Fisher, 44.

Murari Sharma, managing director of Everest Parivar Expedition Pvt Ltd, said Fisher and his Sherpa guide reached the summit at around 8:30 .a.m. on Saturday and had descended 150 metres when he fell unconscious. A group of Sherpas changed his oxygen bottle and tried to give him some water but he could not be revived, he said.

Sherpa tribespeople were mostly yak herders and traders living deep within the Himalayas until Nepal opened its borders in the 1950s. Their stamina and familiarity with the mountains quickly made them sought-after guides and porters.

About 5,000 people have scaled the Everest summit so far and about 300 have died on its slopes.

Another climber was confirmed to have died Monday while descending from the summit. (Phurba Tenjing Sherpa/Reuters)