Almost 250 rescue workers, medical team members, police officers and military personnel backed by eight helicopters and a dozen search dogs were deployed after the avalanche on Plaine Morte, a mountain in the town of Crans-Montana, officials said.
He said the avalanche was 840 meters (yards) long and 100 meters wide, with 400 meters of heavy snow hitting the piste.
"Several people are buried", Valais cantonal police said on Twitter.
Nicolas Féraud, mayor of Crans-Montana, told local newspaper Le Nouvelliste that between 10 and 12 people could be trapped, but emphasized that was an estimate.
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"Witnesses told us there could be more people buried under the snow, that is why we are pursuing the search with considerable means".
Tuesday's snow slide was rare in that it struck a designated ski area, while the majority of deadly avalanches occur off-piste.
Valais prosecutor Catherine Seppey said authorities were investigating what triggered the avalanche, citing weather conditions or skier behavior as possibilities.
Police say rescue teams are at the scene to assist those caught in the avalanche. "Several people are buried".
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Plaine-Morte, at an elevation of about 3,000 meters (9,800 feet), is the highest ski track at Crans-Montana. "It's a spring avalanche which is very compact", the Nouvelliste quoted one unnamed rescue worker as saying.
Search dogs and at least two rescue helicopters are reportedly at the site.
It coincided with the school holidays in some of Switzerland's cantons (states) and other countries in Europe. The resort was scheduled to host two women's races this weekend.
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