At least six people were killed and an indeterminate number are missing.

Rescuers on Sunday spotted six bodies and said nine injured survivors were found. Attention was focused on determining how many people might have been hiking on the Marmolada peak and are unaccounted for. Sixteen cars remained unclaimed in the area's parking lot.

Authorities were trying to track down occupants through vehicle license plates. The injured were flown by helicopters to hospitals in northeastern Italy.

After the search was temporarily halted on Sunday night, officials said about 15 people could be missing, but stressed the situation was evolving.

Rescuers said conditions downslope from the glacier, which has been melting for decades, were still too unstable to immediately send teams of people and dogs to dig into tons of debris.

Premier Mario Draghi and the head of the national Civil Protection agency were expected to go on Monday to Canazei, a tourist town in the Dolomite range which has been serving as a base for rescuers.

Relatives were also expected to go to the town to identify bodies when rescuers can safely remove them from the mountain.

What caused a pinnacle of the glacier to break off and thunder down the slope at a speed estimated by experts at some 300 kph wasn't immediately known. But the heat wave gripping Italy since May, bringing temperatures unusually high for the start of summer even up in the normally cooler Alps was being cited as a likely factor.

Jacopo Gabrieli, a polar sciences researcher at Italy's state-run CNR research centre, noted that the long heat wave, spanning May and June, was the hottest in northern Italy in that period for nearly 20 years.

He said it would have been impossible to predict when or if a serac -- a pinnacle from a glacier's overhang -- could break off.

Alpine rescuers on Sunday noted that late last week, the temperature on the 3,300-metre high peak had topped 10C, far higher than usual. Operators of rustic shelters along the mountainside said temperatures at the 2,000-metre level recently reached 24C, unheard-of a place where excursionists go in summer to keep cool.

The glacier, in the Marmolada range, is the largest in the Dolomite mountains in northeastern Italy. People ski on it in the winter. But the glacier has been rapidly melting away over the past decades, with much of its volume gone. Experts at Italy's state-run CNR research centre, which has a polar sciences institute, estimated a couple of years ago that the glacier won't exist anymore within 25-30 years.

The Mediterranean basin, which includes southern European countries like Italy, has been identified by UN experts as a "climate change hot spot," likely to suffer heat waves and water shortages, among other consequences.

Australian Associated Press