KATHMANDU, Nepal: A crevasse strewn with chunks of ice, including one towering block, has stranded hundreds of mountaineers at the Himalayan base camp for Mount Everest, before they start their climb to the world's tallest peak, officials and climbers said.

The climbing season for Mount Everest, which stands at 8,849 meters (29,031.7 feet), runs from April to May, when the weather is best for reaching the cold, windy summit.

This year, however, a huge 100-foot block of ice (called a serac) is blocking the route above base camp.

Experienced Sherpa climbers, known as "icefall doctors," have not yet been able to fix ropes, set up ladders, or create a safe path through the dangerous Khumbu Icefall to Camp II. As a result, climbers who are eager to begin while the weather is good have to wait.

Garrett Madison of Madison Mountaineering said all expeditions are delayed because of this problem. He added that the ice block could fall soon, but it might still take time. He is leading a team on his 16th Everest climb.

Usually, the route opens by the third week of April, according to Himal Gautam from Nepal's tourism department.

If the ice collapses or melts soon, the Sherpa team can open the route, and climbers may still stay on schedule. If not, more Sherpas may be sent to assess risks and identify alternative routes. Currently, eight "icefall doctors" are at base camp.

Mingma Sherpa, who has reached the summit 11 times, said that practice climbs and the move of equipment to higher camps have been delayed.

Everest lies between Nepal and Tibet, and can be climbed from both sides, though most climbers go from Nepal.

Climbing generates significant income for Nepal, which has eight of the world's 14 highest peaks. So far, 410 climbing permits have been issued this season, each costing US$15,000. There is no limit on permits, which has raised concerns about overcrowding in the "death zone," where oxygen levels are very low.

Among those waiting are 98 climbers from China (including 24 women), the highest number from any country so far. There are also 49 climbers from the United States and 46 from India.

Lukas Furtenbach of Furtenbach Adventures, who is leading 40 climbers, said more people may come from the Nepal side this year because China has closed the Tibetan side for unknown reasons.