The cause of a Russian plane crash in Egypt is looking more like an explosion but it is not clear whether it was linked to a fuel or engine trouble or a bomb, an Egyptian source close to the investigation said on Wednesday.

The Airbus A321 crashed on Saturday shortly after taking off from the resort of Sharm el-Sheikh on its way to the Russian city of St Petersburg, killing all 224 people on board.

"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," the source, who is close to the team investigating the black boxes, told Reuters.

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

Islamic State, which controls swathes of Iraq and Syria and is battling the Egyptian army in the Sinai Peninsula, said again on Wednesday it brought down the airplane and said it would eventually tell the world how it carried out the attack.

Egypt dismissed a similar statement by Islamic State on Saturday.

Security experts and investigators have said the plane is unlikely to have been struck from the outside and Sinai-based militants are not believed to possess the technology to shoot down a jet from a cruising altitude above 30,000 feet.

Any evidence that a bomb knocked the plane out of the sky would raise questions over Egyptian President Abdel Fattah al-Sisi's assertions that Egypt had brought under control the insurgency waged by the Islamic State affiliate, Sinai Province.

SISI VISIT

Britain said on Wednesday the airplane may have been brought down by an explosive device. But Prime Minister David Cameron's office was cautious over the possible causes.

"While the investigation is still ongoing we cannot say categorically why the Russian jet crashed," it said in a statement.

"But as more information has come to light we have become concerned that the plane may well have been brought down by an explosive device."

Britain's assessment of the crash came during a visit to London by Sisi.

Russian President Vladimir Putin's spokesman Dmitry Peskov said on Monday that no theory could yet be ruled out.

If the crash was the result of sabotage, it would almost certainly deal a heavy blow to tourism in Egypt, which is highly sensitive to attacks by Islamist militants on foreigners.

The industry, a pillar of the economy, was already struggling to recover from years of political turmoil triggered by an uprising that toppled autocrat Hosni Mubarak in 2011.

As a precautionary measure, Cameron's government has decided that flights due to leave Sharm for Britain on Wednesday evening would be delayed to allow time for a team of UK aviation experts, currently travelling to Sharm, to make an assessment of the security arrangements in place at the airport.

Sisi, who as army chief toppled Islamist President Mohamed Mursi in 2013 after mass protests against his rule, was elected president last year on promises he would stabilise Egypt and rebuild a limping economy. - Reuters