Rescuers using drones have resumed the search for an estimated 13 hikers who are unaccounted for following an avalanche in northern Italy.

At least seven people died in the tragedy which is being blamed in large part on rising temperatures that are melting glaciers.

After rain hampered the search on Monday, sunnier weather allowed helicopters to bring more rescue teams up to the site on the Marmolada glacier, east of Bolzano in the Dolomites mountain range.

A huge chunk of the glacier broke off on Sunday, sparking an avalanche that sent torrents of ice, rock and debris down the mountainside onto unsuspecting hikers below.

At least seven people were killed and an estimated 13 remain unaccounted for, officials said.

The terrain is so unstable that rescue crews are staying off to the side and using drones to try to find any survivors while helicopters searched overhead, some using equipment to detect mobile phone pings.

Two rescuers remained on site overnight, and were joined by more on Tuesday morning.

"We're continuing the work of drones to find survivors, working the areas that we couldn't monitor yesterday," Matteo Gasperini, of the Alpine Rescue service, told Sky TG24.

"We'll try to complete the work of monitoring the entire site."

Italian Premier Mario Draghi, who visited the rescue base in Canazei on Monday, acknowledged avalanches are unpredictable but that the tragedy "certainly depends on the deterioration of the climate situation".

Italy is in the midst of an early summer heatwave, coupled with the worst drought in northern Italy in 70 years.

Experts say there was unusually little snowfall during the winter, exposing the glaciers of the Italian Alps more to the summer heat and melt.

"We are thus in the worst conditions for a detachment of this kind, when there's so much heat and so much water running at the base," said Renato Colucci from the Institute of Polar Sciences of the state-run Council for National Research (CNR).

"We aren't yet able to understand if it was a deep or superficial detachment, but the size of it seems very big, judging from the preliminary images and information received."

The CNR has estimated that the Marmolada glacier could disappear entirely in the next 25-30 years if current climatic trends continue, given that it lost 30% of its volume and 22% of its area from 2004-2015.