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 was unstable and risky for climbers, said Himal Gautam of Nepal’s department of mountaineering on Friday.

Officials were working with climbers and expedition organisers to assess the situation as climbers waited at base camp.

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

“Icefall doctors”, elite guides 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, said it planned to assess the serac by aerial survey. The risk of avalanche was high and they were waiting for the serac to melt down 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-storey buildings. It is considered one of the hardest 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 reached the summit of the 8,849-metre (29,032-ft) peak since it was first reached on 29 May 1953 by the Sherpa guide Tenzing Norgay and the New Zealander Edmund Hillary.