French authorities said four avalanches were triggered in the Alpine locations Val Thorens, La Plagne, Les Arcs and Val d'Isere, killing three, including a woman, and injuring a fourth who later died.

One snowboarder skiing off-piste was buried when a slope collapsed under him.

A Swedish man was among the dead, officials in Stockholm said, adding that he died in Val Throens.

Authorities issued a warning of increased avalanche danger following recent heavy snowfall, as a cold snap gripped much of western Europe with temperatures dipping below zero and snow and ice affecting traffic in many areas.

Swiss police in the Valais canton said hopes were dwindling of finding the two Canadians alive 48 hours after they disappeared in an avalanche.

Heavy fog had made it impossible Tuesday for helicopters to search the area where they went missing, and the ground was too dangerous for a rescue team to set out on foot.

Despite avalanche warnings in Switzerland, the Canadian pair, a 27-year-old man and his wife, 26, left marked ski areas in Crans-Montana in the central Valais region with two other snowboarders on Sunday morning, police said.

They triggered a wave of snow as they headed off piste down the Ertentse valley and were swept away in the cascade. Their two companions managed to escape and raise the alarm.

A helicopter patrolled the area Monday in search of the couple but made no sightings, emergency officials said.

"If an avalanche victim is not rescued within 45 minutes, the possibility of survival is small, after three hours it is very small," said Jean-Marie Bornet, head spokesman for the Valais police.

A Davos-based Swiss institute specialising on study of snow and avalanches meanwhile warned that an avalanche danger remained acute in the Valais area.

Some villages in Switzerland's Bernese Oberland and the fasionable resort of Zermatt were Tuesday reachable only by train, with the main access road closed until further notice.

There were several avalanches across the Alps over the weekend.

A US skier was reported missing Sunday following avalanches in western Austria that also killed three people and injured four others, police said.

At Sankt Anton, an avalanche on Saturday swept away a group of four Canadian skiers along with their Austrian instructor and another group of four Americans following them on an off-piste trail.

Police said two Canadians and an American were among the dead.

Traffic around Germany was held up in several regions as snow blocked roads and ice made driving difficult, police said, with some 70 weather-related traffic jams across the country.

Temperatures also dipped in France, prompting the government to declare an alert calling for more space in homeless shelters.

Snow was reported in the northwest of the country, and local authorities in Normandy called on residents to limit their travel and to signal any homeless people left out in the cold.

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