President Xi Jinping sent a message of condolence on Monday to his Russian counterpart Vladimir Putin after a passenger plane with 71 people on board crashed just outside Moscow on Sunday.

In the message, Xi said he was shocked after learning of the crash, which claimed the lives of all the passengers and crew members.

Xi expressed condolence toward those who lost their lives and sympathy for the victims' families on behalf of the Chinese government and people, as well as himself.

On the same day, Premier Li Keqiang also sent a condolence message to Russian Prime Minister Dmitry Medvedev.

At the crash site, investigators were carrying out the grim task of looking for clues and the remains of victims.

The site of the crash was enveloped in heavy snow that was waist-high in places, making it difficult to access, with emergency workers forced to reach the wreckage by foot and use snowmobiles.

Putin, who offered "his profound condolences to those who lost their relatives in the crash", has ordered a special commission to investigate what caused the AN-148 plane operated by Saratov Airlines to crash outside Moscow shortly after taking off for the city of Orsk about 1,500 kilometers southeast of the capital.

Russia's Investigative Committee said it would consider explanations for the crash including human error, technical failure and weather conditions, as the country has experienced record snowfall in recent weeks. It did not mention the possibility of terrorism.

"Sixty-five passengers and six crew members were on board, and all of them died," Russia's office of transport investigations said in a statement.

A Swiss citizen and a citizen of Azerbaijan were among the fatalities on a list released by the emergency services ministry. Three children also died including a 5-year-old girl.

Mourners left flowers, soft toys and photographs at Orsk airport on Monday.

"We believed until the very end yesterday that at least someone would survive," said a distraught Ekaterina Zakladnaya, 20, whose mother Natalia was killed in the crash.

"We phoned the airports and barely managed to get through and then they published the lists and mummy was second from the bottom."

Around 100 investigators and criminologists were working at the scene, the Investigative Committee said.

The emergency services ministry said one of the two black boxes had been found.

With wreckage of the plane spread over more than 30 hectares around the crash site, it will take a week to inspect the whole area, the emergency services ministry said.

More than 900 people using equipment including drones were involved in the search operation, which has been reclassified as looking for bodies rather than survivors, the ministry said.

"We plan to carry out the main stage of the search operation in seven days because the plane debris is scattered over a very large area," emergency services minister Vladimir Puchkov said at the scene, quoted by Interfax news agency, adding that "heavy snow" hampered searchers.

"We walked about 600 to 700 meters across a field, with snow in places waist-deep," said Alexei Besedin, one of the first rescuers to reach the scene, quoted by the emergency services ministry.

The transport investigations office said the plane disappeared from radar screens several minutes after take-off.

The Russian-made plane was reportedly 7 years old and bought by Saratov from another Russian airline a year ago.

China Daily - Afp - Reuters