A large ice block above Mount Everest Base Camp has forced hundreds of climbers and local guides to suspend their ascent toward the summit. Nepal’s Department of Mountaineering official Himlal Gautam said on Friday that the ice barrier, or serac, between Base Camp and Camp One was unstable and unsafe for passage. Climbers are currently waiting at the base camp while authorities monitor the situation.

According to government data, 410 foreign climbers have been granted permits to attempt Everest this spring season, which typically ends in late May. Experienced Sherpas, known as “Icefall Doctors,” usually complete rope and ladder installations across the Khumbu Icefall by mid-April, but this year the work has been delayed due to the unstable serac. The Sagarmatha Pollution Control Committee plans to conduct an aerial survey to assess the risk, with its chairman Lama Kaji Sherpa warning that avalanche danger remains until the ice melts to a safer level.

The Khumbu Icefall is one of the most perilous sections of the Everest route, and a similar ice collapse in 2014 killed 16 Sherpa guides, marking one of the deadliest incidents in the mountain’s history.