Ten backcountry skiers are missing following an avalanche in northern California on Tuesday, with a search and rescue operation underway amid “highly dangerous” conditions, authorities said.

The avalanche was reported midday Tuesday in the Castle Peak area in the Sierra Nevada, according to the Nevada County Sheriff’s Office.

A group of 16 skiers — including four ski guides and 12 clients — encountered the avalanche, according to the sheriff’s office.

Six people survived and were awaiting rescue at the avalanche site, while 10 remained unaccounted for, the sheriff’s office said.

Rescuers faced very difficult conditions, including avalanche danger themselves, according to Brandon Schwartz, director of the Sierra Avalanche Center, which forecasts avalanche conditions in the area around Lake Tahoe. The area saw 2 to 3 feet of new snow in the last 36 hours and more was still falling at 2 to 4 inches per hour, Schwartz told ABC News.

The Blackbird Mountain Guides said the avalanche happened near the Frog Lake huts in the Castle Peak area. The group of skiers had been staying at the huts since Sunday.

“The group was in the process of returning to the trailhead at the conclusion of a three-day trip when the incident occurred,” the organization said in a statement.

The Sierra Avalanche Center said there was “high” avalanche danger in the backcountry on Tuesday.

“Rapidly accumulating snowfall, weak layers in the existing snowpack, and gale-force winds that blow and drift snow have created dangerous avalanche conditions in the mountains,” the center said. “Natural avalanches are likely, and human-triggered avalanches large enough to bury or injure people are very likely.”

The Sierra Avalanche Center has issued an avalanche warning for the Central Sierra Nevada Mountains between Yuba Pass on the north and Ebbetts Pass on the south, including the greater Lake Tahoe area, through Wednesday morning.

“Large avalanches are expected to occur Tuesday, Tuesday night, and into at least early Wednesday morning across backcountry terrain,” the center said, advising against travel in, near or below backcountry avalanche terrain. “HIGH avalanche danger might continue through the day on Wednesday.”

Whiteout conditions have been reported in the region where the avalanche occurred.

The California Highway Patrol’s Truckee office warned that high winds are “creating full whiteout conditions” across the Donner Summit. 

Interstate 80 over Donner Summit was closed in both directions on Tuesday due to whiteout conditions and poor visibility.