An anonymous source told the Russian news agency Interfax: “Sounds uncharacteristic of routine flight were recorded preceding the moment that the aircraft disappeared from radar screens.”
Interfax said the recordings, from the plane’s black boxes, suggested “there was a sudden emergency situation on board which took the crew by surprise, and the pilots did not have time to send a distress signal”.
Egypt’s civil aviation ministry has disputed Russian claims that the airliner broke up in mid-air. Spokesman Mohamed Rahmi said there was no proof this had happened, adding: “This could be a long process and we can’t talk about the results as we go along.”
On Monday, NBC News reported that US officials believed an explosion on board had caused the crash.
Mr Rahmi confirmed that a distress call had not been made from the Airbus 321 before it came down.
He said investigators had returned to the crash site on Tuesday morning and that once their work there had finished, they would focus on analysing the contents of the black boxes.
The plane had been flying back Russian holidaymakers from the Egyptian resort of Sharm el-Sheikh to St Petersburg.
The fact there was no distress call and debris is scattered over a wide area suggests there was a sudden break-up of the plane, according to some experts. Possible causes include mechanical failure, pilot error, a bomb or being hit by a missile fired from the ground.
A Sinai-based Egyptian militant group allied to Islamic State claimed on Saturday to have shot down the plane in response to Russia’s military intervention in Syria in support of President Bashar al-Assad, a suggestion dismissed as “propaganda” by Egyptian President Abdul Fattah al-Sisi in an interview with the BBC.
Islamist militants in Sinai are not thought to have the military hardware that would enable them to bring down a plane at 30,000 feet, the altitude the Airbus 321 was flying at before it was hit.
However a source analysing the black boxes told Reuters on Monday the plane had been hit from the outside.
All of the victims’ bodies have now been returned to Russia.