The skiers who died in an avalanche in California's Lake Tahoe backcountry are said to have been part of a tight-knit group who often went on ski excursions together.

Eight people died after an avalanche struck near Castle Peak in the Sierra Nevada mountains on Tuesday, and rescue teams are searching for a ninth who is presumed dead. Six members of the 15-person ski group have been rescued.

The "football-field sized" avalanche, which was the deadliest in modern California history, hit during the popular "ski week" holidays in the state.

Sisters Caroline Sekar, 45, of San Francisco, and Liz Clabaugh, 52, of Boise, Idaho, were among those who died, their brother told the New York Times.

McAlister Clabaugh, 50, said that many in the group of 11 skiers had been friends since university and would regularly meet for ski trips.