Some 17 people remain unaccounted for a day after a huge chunk of an Alpine glacier broke off and crashed into hikers in northern Italy, officials said.

At least six people died and nine were injured as the avalanche of ice, snow and large rocks thundered down the slope of the mountain topped by the Marmolada glacier on Sunday afternoon.

Trento prosecutor Sandro Raimondi said that 17 hikers were believed to be missing, the Italian news agency LaPresse reported.

Veneto regional governor Luca Zaia said some of those hiking in the area on Sunday were roped together as they climbed.

Nationalities of the known dead have not been disclosed, and conditions were too dangerous on Monday morning for rescue crews with dogs to resume the search for those missing or to bring down the bodies.

The bodies will be brought to an ice skating rink in the resort town of Canazei in the Dolomite mountain range for identification.

Mr Raimondi was quoted as saying two of the nine injured are Germans. Mr Zaia told reporters that one of the Germans was a 65-year-old man.

Of the injured patients, one of those in intensive care has yet to be identified.

The patients suffered chest and cranial injuries, said Mr Zaia.

Drones were being used to look for any of the missing.

Sixteen vehicles remain unclaimed in the area's car park, and authorities sought to track down occupants through licence plates.

It is unclear how many of the cars might have belonged to the already identified victims or to the injured, all of whom were flown by helicopters on Sunday to hospitals.

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

Premier Mario Draghi and the head of the national Civil Protection agency were traveling to the stricken area for briefings.

What caused a pinnacle of the glacier to break off and thunder down the slope at a speed estimated by experts at nearly 200mph was not immediately known.

But the heatwave 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 heatwave, spanning May and June, was the hottest in northern Italy in that period for nearly 20 years.

"It's absolutely an anomaly," Mr Gabrieli said in an interview on Italian state TV.

Like other experts, he said it would have been impossible to predict when or if a serac - a pinnacle from a glacier's overhang - could break off, as it did on Sunday.

Alpine rescuers on Sunday noted that late last week, the temperature on the 3,300-metre high peak had topped 10C (50F), far higher than usual.

Operators of rustic shelters along the mountainside said temperatures at the 2,000-metre level recently reached 24C (75F), unheard-of heat in a place where visitors go in summer to keep cool.

The glacier, in the Marmolada range, is the largest in the Dolomite mountains in north-eastern 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 will not exist within 25-30 years.

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