Two people were killed and two remained missing after avalanches in Colorado, authorities said Monday, warning that conditions remain dangerous in the western US state.

One person was killed on a closed ski run in Vail Mountain, 100 miles west of Denver, on Sunday, while another death was reported in the Winter Park ski area, also west of the city, sadi the Colorado Avalanche Information Center.

Meanwhile two snowmobilers were missing after a reported avalanche accident in the east side Buffalo Pass, 120 miles northwest of Denver, it said in a statement on its website.

"Rescue efforts began yesterday and are still in progress," it said.

And it warned: "Widespread dangerous avalanche conditions exist today. Triggering avalanches is likely on any snow-covered slope 30 degrees or steeper that did not slide during the natural cycle yesterday.

"The natural avalanche cycle has largely run its course but natural avalanches are still possible today. Triggering slides will be easy today, and some of them will be bigger than what we have seen so far this winter."

Colorado is famous for its Rocky Mountains ski-ing, notably in the high-end resort of Aspen and in Beaver Creek, where Alpine Ski Men's World Cup races were held in December.