Recordings from the Metrojet flight that crashed in the Egyptian desert have revealed strange sounds immediately before contact was lost, according to a Russian news agency.

The plane, carrying 224 people, went down nearly 25 minutes after it left Sharm el-Sheikh in Egypt to Saint Petersburg, Russia, early on Saturday morning.

Pilots of the aircraft made no distress call before the plane lost contact, Ukrainian news agency Interfax reported.

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It said recordings indicated normal operations four minutes before it lost communication, before foreign noises were heard.

“The crew was engaged in routine communications with air traffic controllers,” the news agency said, attributing the details to a source in Cairo, The Sydney Morning Herald reported.

“Sounds uncharacteristic of a standard flight precede the moment of the airliner’s disappearance from radar screens.

“The recordings suggest that an emergency situation occurred on board unexpectedly, took the crew by surprise and the pilots had no time to send out a distress signal.”

The Interfax report was unverified.

On Tuesday, military experts said it was likely an explosive device – either with a timer, or triggered by altitude – had been on-board the aircraft before it left the airport.

A US infrared satellite detected a heat flash over the Sinai peninsula at the time the crash occurred, CBS News reported.

Another unverified report was published by Russian news agency LifeNews, that claimed the bodies of the passengers seated in the rear of the plane showed signs of blast injuries.

Those in the front showed typical trauma injuries.

Human error or a technical fault had been ruled out as causes, according to Russian airline Kogalymavia, the operator of the Metrojet Airbus A321 flight.

It was also becoming unlikely – although not impossible – Islamic State, who claimed responsibility for the downing of the plane soon after the attack, could have shot the plane down from the ground.

Speaking to the media on Monday, Metrojet deputy general director Alexander Smirnov confirmed the plane slowed by 300 km per hour and dropped about 5000ft (1500m) in altitude a minute before it crashed, having just reached its cruising altitude of around 32,000ft.

“This isn’t flying, it’s falling. Apparently, the plane sustained damage before this [and] that became the reason for the fall,” Mr Smirnov said, according to The Guardian.