Strong winds were blamed as nine climbers, including three Britons, were killed and seven injured in an avalanche on the Col de Maudit, near the French Alps' renowned ski resort of Chamonix.
The victims were swept away by an early morning avalanche which came down before 5.30 am local time.
Two other Britons are reported missing following the avalanche on the slopes of Mont Maudi.
Up to 40 rescuers, assisted by helicopters, are searching for the missing. In all, 28 climbers were roped together in several groups.
They are believed to have reached 4,000m (13,120 ft) when the avalanche struck.
The mountaineers were on their way up the Mont Blanc on the Trois Monts route (Mont-Blan du Tacul, Mont-Maudit, Mont-Blanc).
The cause behind the avalanche could be strong winds overnight provoking a wind slab on the lee slope.
The climbers are believed to have been roped together in several groups when the avalanche struck.
Mont Maudit, which in French means "Cursed Mountain", is the third-highest peak in the Mont Blanc massif range.