authorities say. Six others have been rescued after they became stranded, the Nevada County Sheriff's Office said. The avalanche was reported about 11:30 PST (19:30 GMT) in the Castle Peak area near the town of Truckee, according to officials. Weather conditions remain dangerous, with a high chance of further avalanches. "Highly skilled rescue ski teams" have deployed from two nearby ski resorts, the sheriff said. The six known survivors have been "directed to shelter in place as best they can in the conditions," the sheriff added.

Officials initially reported that a group of 16 people in total - four guides and 12 clients - were involved, though that number was later lowered to 15. "The six initially reported skiers who survived the avalanche have been successfully rescued this evening," the sheriff's office wrote on Facebook late on Tuesday night. Two of the six rescued skiers were taken to hospital, it said. Experts from the nearby Boreal Mountain Ski Resort and the Alder Creek Adventure Center are also attempting to reach the site. An earlier statement posted by the sheriff's office said 46 emergency responders had joined the rescue efforts. The Sierra Avalanche Center wrote in a forecast: "Large avalanches are expected to occur [on] Tuesday, Tuesday night, and into at least early Wednesday morning across backcountry terrain." The avalanche centre listed the danger rating as "high" - a four out of five on the rating scale. "Travel in, near, or below avalanche terrain is not recommended today. A widespread natural avalanche cycle is expected over the next 24 hours," the centre said. "Large avalanches may run through treed areas." The avalanche that trapped the skiers was rated as a D2.5 on a scale of D1 to D5, Sierra Avalanche Center Forecaster Steve Reynaud told local outlet ABC News 10. D2 avalanches can injure, bury or kill people, according to avalanche.org. (BBC)