A six-year-old girl, who was found unconscious in a forest after she disappeared during a family party, has died, police in Switzerland said on Monday.

Police in Lausanne in western Switzerland were informed of the girl's disappearance on Sunday at 5:54pm (15:54 GMT) while she was attending a family party in the Ravoire region.

She had not been seen by anyone since 5:00pm when she was playing at the party.

"The six-year-old year was taken into critical care by an Air-Glaciers helicopter to Lausanne University Hospital (CHUV) in Lausanne where she unfortunately died" late on Monday morning, the Valais police said in a statement.

The cantonal police, in collaboration with several rescue organisations, the air force and more than 200 volunteers, had launched a major search to locate the missing girl.

On Sunday evening, at 9:20pm, a search in the forest close to the gathering led to the discovery of the unconscious girl.

The cantonal prosecutor's office has opened an investigation into the circumstances behind the incident.

Authorities did not provide any further comment on the investigation.