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Italian authorities have evacuated about 75 people, mostly tourists, from an Alpine valley as huge blocks of ice threaten to crash down from a glacier.

Planpincieux glacier, in the Mont Blanc massif, has weakened because of intense summer heat alternating with night-time cold. It lies above Val Ferret valley, near Courmayeur ski resort.

A local environmental risk expert said the fragile ice could fall at any time.

The threatening glacier section is about the size of Milan cathedral.

The risk manager, Valerio Segor, said "the water flowing underneath can, in fact, act as a slide" and they faced "the risk of immediate collapse".

The fragile 500,000 cubic metres (18m cu ft) of glacier is being monitored with aerial photography and radar.

Roads leading to Val Ferret, a popular area for hikers, have been closed off.

A similar alert and evacuation took place last September, because of the unusually hot Alpine summer, attributed to global warming.

The glacier is at a height of 2,600-2,800 metres (8,500-9,200 ft). The Mont Blanc massif is the highest mountain in western Europe, at over 4,800m.

A Courmayeur official, Moreno Vignolini, said the heatwave had accelerated the glacier's melting rate, pushing it as high as 50-60cm (16-23in) a day.