AKIPRESS.COM - Military experts say a small bomb — either with a timer, or triggered by altitude — now appears to be the most likely cause of the Russian plane crash in Egypt's Sinai desert.

Charles Heyman, editor of annual publication Armed Forces of the United Kingdom, said most analysts had begun to think there was an explosion on board the aircraft, reports ABC News.

"Maybe not a huge explosion, but an explosion big enough to actually cause the aircraft to shatter, explode in mid-air and then deposit bits of the aircraft over a large area of ground," Mr Heyman said.

He said even a very small bomb smuggled inside someone's hand luggage would have enormous force at such altitude because the cabin was pressurised.

Russian airline Kogalymavia has said the Metrojet Airbus A321 came down from about 30,000 feet because of "external" factors, with human error or a technical fault now ruled out as the cause.

The carrier's management confirmed no emergency call was made by the pilots during the flight, saying the crew "totally lost control" and did not attempt to make any contact.

At a press conference, Kogalymavia deputy general director Alexander Smirnov said "some kind of external action" was "the only explanation".