At least 12 climbers died and five are missing and feared dead on the 8,849 metre (29,032 foot) Mount Everest during this year’s main climbing season that has just ended, the deadliest since an earthquake triggered an avalanche that killed 18 people in 2015.

“Weakest clients with less experienced operators is part of the problem,” Guy Cotter, 69, a noted guide from the New Zealand who has climbed Everest five times, said in the Nepali capital, Kathmandu.

“Clients should have minimum climbing standards with proven prior ascents before coming to Everest,” he said.

Three sherpa climbers were killed when ice came crashing down on them on the lower reaches of the mountain in April and the rest of those who died succumbed to illness or exhaustion, government and hiking officials said.

Nepal does regulate the climbing of Everest and its other Himalayan peak, insisting that everyone gets a permit, for example, but Cotter said the operators who guide clients up the mountain should meet minimum standards of equipment and staff.

“There are many operators who take clients to Everest but do not understand how to avoid incidents occurring and when things do go wrong they do not have processes in place to address the problems,” Cotter said.

Bigyan Koirala, an official with the Department of Tourism that oversees climbing, said the government was considering more regulations but did not give details.

Nepal issued a record of 478 permits for Everest this year and hundreds of people made it to the summit.

Garrett Madison, 44, who climbed the 8,849 metre (29,032 feet) Everest for the 13th time last week, said its higher camps were littered with torn tents, food wrappers and empty oxygen bottles discarded by climbers.

“We need to find better ways to bring the waste down,” Madison said in the Nepali capital Kathmandu after returning from the mountain.

“We need better policing to check that every team brings down its garbage.”

It is mandatory for climbers to bring their waste down from the mountain and claim back a garbage deposit of $4,000 from the government.

But expedition organisers and hiking officials say monitoring camps nearly 8,000 metres (26,246 feet) high was both difficult and ineffective.

Authorities collected 13 tonnes of rubbish from Everest and the nearby Lhotse peak this year as part of a campaign to keep the mountains clean.

Despite his worries about the trash, Madison, who owns a Seattle-based mountaineering company, said climbing in Nepal, home to eight of the world’s 14 highest mountains, had a bright future.

“I think Nepal is the Switzerland of Asia in its potential to develop mountaineering,” he said, adding that the country had better emergency helicopter services for climbers than Pakistan and the Tibet region, where the six other highest peaks are located.

This month, as well as Everest, Madison climbed Lhotse, the world’s fourth tallest peak at 8,516 metres (27,939 feet), and the Nuptse peak, at 7,855 metres (25,770 feet), completing the rare “triple crown” of climbing all three in one season.

Mountain climbing generates big income for Nepal, which issued a record 478 permits for Everest this March to May season, each costing $11,000.

While hundreds of people climbed the mountain this season, 12 of them died and five were missing on its slopes.

Pasang Dawa Sherpa reached the summit on Sunday morning along with a Hungarian climber, according to expedition organizer Imagine Nepal Treks.

The season’s first wave of climbers reached the summit this weekend as Sherpa guides fixed ropes and made paths for the hundreds of climbers who will attempt to scale the peak over the next few weeks.

Since making his first successful of climb of the peak in 1998, Dawa has made the trip almost every year.

Experienced mountain guide Kami Rita earlier held the record outright for the most climbs of Mount Everest after his 26th successful trip last year. Rita is expected to attempt to climb the peak again later this month as he guides foreign climbers to the top of the world.

Climbers generally reach the base camp of the mountain in April and spend weeks acclimatizing to the high altitude, rough terrain and thin air before they go up the mountain’s slopes. By the first or second week of May, they are usually making attempts for the summit.

This year’s climbing was slightly delayed after three Sherpa climbers fell into a deep crevasse on a treacherous section of the mountain in April. Rescuers have not been able to find them.

With the opening of the route to the summit, a rush to make their attempts is expected in the next couple of weeks as the Nepalese authorities have issued nearly 470 permits for Everest during the popular spring climbing season.

This year also marks the 70th anniversary of the first ascent of Mount Everest in 1953 by New Zealander Edmund Hillary and his Sherpa guide Tenzing Norgay.