Four Russian climbers have reportedly fallen ill with an unknown illness while attempting to climb Mount Everest, with one member of the group evacuated by helicopter.

Climbers including Andrey V. began their ascent on October 19. Natalia B. was the first to develop symptoms, including weakness and dizziness. Unable to continue, she was later airlifted off the mountain. The team has not determined what caused her illness, according to SHOT.

The remaining climbers continued their ascent without supplemental oxygen or support. By the fourth day, all of them had developed similar symptoms.

The group is now attempting to descend, but severe weather has made the usual walking routes in the area inaccessible. There are also no medical personnel nearby, leaving the climbers with limited access to urgent assistance.

Extreme cold and worsening weather are adding to the danger. Winds are expected to strengthen over the next 24 hours, with October 23 forecast to be particularly hazardous.