The avalanche struck very early in the morning in the town of Farindola, in the lower Gran Sasso mountain range.
It followed days of heavy snow, and is believed to have been triggered after four earthquakes over magnitude-five shook Abruzzo province in central Italy.
Guests at the luxury ski resort, Hotel Rigopiano, were reportedly gathered on the ground floor to await evacuation when the avalanche hit.
The view from a helicopter surveying the area shows no buildings, roads or other signs of civilisation - just a thick and untouched coat of fresh snow.
The first rescuers could only reach the hotel on skis, a full day after the avalanche.
A local government official, Carlo Torlontano, says severe weather conditions have hampered their efforts.
"We are doing a briefing on the situation. Our vehicles and equipment reached the area two hours ago and they are operating. It was very difficult to get there, we had to basically open a tunnel in the snow, and the diameter of the tunnel was only of the diameter of the leading vehicle. We are now working to enlarge the passage and to create a plain space in front of the hotel for the vehicles to be able to move and work. We are also waiting for about ten soldiers who specialise on avalanche searches. They have special equipment that will try to locate people under the snow."
On site, the roof of the four-storey building has collapsed and locals who know the area say the hotel appears to have shifted 10 metres off its foundation.
Video shows hotel interiors choked with walls of debris and snow.
A list of 23 names has been published in Italian newspapers.
Most of the trapped are believed to be Italians but they also include a Swiss national and a Romanian.
But rescue official, Mario Mazzocca, says they believe more might be missing.
"At the moment we have an undefined number of missing people. In fact, besides the people registered by police as guests of the hotel, there may have been more unregistered guests. We believe there are roughly around 30 to 35 missing people."
The head of the Italian Civil Protection, Fabrizio Curcio, says two people who were outside the hotel at the time of the avalanche have survived.
"We have constant contact with the people on the ground to check critical weather situations, especially to reach this hotel. It was first reached at 4.30am local time by very courageous men who took two people to safety and continue to help rescue vehicles having trouble arriving. But we are already working to improve the conditions, which are really bad for a technical rescue."
There are a multitude of different faults that cut through the Apennine mountains, making the area prone to tremors and quakes.
Avalanche warnings were issued across the region, which has numerous small ski resorts popular with daily visitors from Rome and urban centres on Italy's east coast.
Italy's Prime Minister, Paolo Gentiloni, has praised rescue workers for their efforts.
"I want to say that we are all holding our breath for what has happened last night with the avalanche that struck the Rigopiano hotel. We are holding our breath as we wait to see developments on the dramatic effects that avalanche has probably had. We saw the videos of rescuers reaching the hotel. These videos bear testimony to the sense of duty of the rescuers but also to the very hard conditions they are operating in."
