A large ice block on the route just above the Mount Everest base camp has forced hundreds of climbers and local guides to delay their attempt to scale the world’s highest peak.

The serac between base camp and camp one is unstable and is risky for climbers, said Himal Gautam of Nepal’s department of mountaineering on Friday.

Officials are working with climbers and expedition organisers to assess the situation as climbers wait at base camp unable to move up the mountain.

According to the department, 410 foreign climbers have been issued permits to attempt to reach the Everest summit during the spring climbing season, which ends at the end of May.

“Icefall doctors”, who lay the yearly climbing route by setting ropes and securing aluminium ladders over crevasses, usually finish the task by mid-April.

The Sagarmatha Pollution Control Committee, which deploys teams to lay the route, plans to assess the serac by aerial survey. The risk of avalanche is high and they are waiting for the serac to melt down on its own to a safe level, the committee’s chair, Lama Kazi Sherpa, said.

The serac is part of the Khumbu Icefall, a constantly shifting glacier with deep crevasses and huge overhanging ice that can be as big as 10-story buildings. It is considered one of the most difficult and trickiest sections of the climb.

In 2014, a chunk of the glacier sheared away from the mountain, setting off an avalanche of ice that killed 16 Sherpa guides as they carried clients’ equipment up the mountain. It was one of the deadliest disasters in Everest climbing history.

Hundreds of foreign climbers and about the same number of Nepalese guides and helpers are expected to attempt to scale the mountain next month when there are a few brief windows of favourable weather.

Thousands of people have climbed the 8,849-metre (29,032-ft) high peak since it was first climbed on 29 May 1953 by the New Zealander Edmund Hillary and Sherpa guide Tenzing Norgay.