Britain's top diplomat said Wednesday that a bomb aboard the Metrojet Airbus A321M could have brought the jet down over Egypt's Sinai desert.

"We have concluded that there is a significant possibility that the crash was caused by an explosive device on board the aircraft," Hammond said after a meeting of the government's crisis response committee chaired by the prime minister.

The so-called "Islamic State" (IS) jihadist group has already claimed responsibility for the destruction of the St. Petersburg-bound flight that had taken off from Egypt's Sharm el-Sheikh airport on Saturday.

Britain has suspended flights to Sharm el-Sheikh as an extra precaution.

"Prove that we didn't bring it down, and how it came down. We will detail how it came down at the time of our choosing," IS said in an audio statement posted online. The claim could not be independently authenticated but if confirmed it would signify the first attack by the group against a commercial airliner.

'IS' claims responsibility

In the message, the voice claimed that IS had downed the plane because of Moscow's recent military intervention in Syria against the extremist group, but Egypt has dismissed that as "propaganda" aimed at damaging Egypt's image.

Meanwhile, two American and one European government official told the Reuters news agency that intelligence reporting is leaning toward a deliberate attack but cautioned there was no conclusive proof yet.

"It is believed to be an explosion but what kind is not clear. There is an examination of the sand at the crash site to try and determine if it was a bomb," said an Egyptian official who is close to the team investigating the crash.

"There are forensic investigations underway at the crash site. That will help determine the cause, to see if traces of explosives are found."

Egyptian investigators have extracted and validated the contents of the flight data recorder, one of two so-called black boxes recovered from the Russian plane, Egypt's Civil Aviation Ministry said. A second black box, which contains the cockpit voice recorder, was partially damaged and much work was required to extract data from it.

jar/jr (Reuters, AFP, AP)