Explosion probably cause of Russian plane crash - Egyptian source

* Source close to investigation points to explosion

* Causes of explosion still unclear

* Britain raises possibility of bomb

* Egypt's Sisi in London for talks with Cameron (Adds graphic, Irish airline authority)

By Ahmed Mohamed Hassan and Gleb Stolyarov

CAIRO/MOSCOW, Nov 4 (Reuters) - An explosion probably caused a Russian plane to crash in Egypt last weekend but it remains unclear whether a bomb or technical problems were to blame, an Egyptian source close to the investigation told Reuters on Wednesday.

The Airbus A321M crashed on Saturday in the Sinai Peninsula (Tel Aviv: PEN.TA - news) 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.

Britain, which hosts Egypt's President Abdel Fattah al-Sisi this week, said an explosive device may well have caused the crash, while CNN quoted a U.S (Other OTC: UBGXF - news) . official as saying a bomb could have been planted on board by militants of Islamic State or one of its affiliates.

"It (Other OTC: ITGL - news) 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 the Egyptian source, who is close to the team investigating the black boxes.

"There are forensic investigations underway 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, adding it would eventually tell the world how it carried out the attack.

Egypt, a close ally of the United States and the most populous Arab country, dismissed a similar statement by the ultra-hardline group on Saturday.

Sisi has described Islamist militancy as an existential threat to the Arab world and the West and has repeatedly called for greater international efforts to combat the militants.

The office of British Prime Minister David Cameron, who was due to hold talks in London with Sisi on Thursday, said in a statement: "As more information has come to light we have become concerned that the plane may well have been brought down by an explosive device."

Striking a similar note, the U.S. official quoted by CNN said: "There is a definite feeling it was an explosive device planted in luggage or somewhere on the plane."

"SOMETHING STOWED" ON BOARD

A Russian aviation official said on Wednesday the investigation was looking into the possibility of an object stowed on board causing the disaster.

"There are two versions now under consideration: something stowed inside (the plane) and a technical fault. But the airplane could not just break apart in the air - there should be some action. A rocket is unlikely as there are no signs of that," the Russian official said.

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.

A Western diplomatic source in Europe said the claim of responsibility by an affiliate of Islamic State, called Sinai Province, was being treated as serious.

"The theory of an explosive device, with local complicity, is being taken seriously. Nothing is proven yet, but it is a real possibility," he told Reuters.

Any evidence that a bomb knocked the plane out of the sky would deal a heavy blow to tourism in Egypt, a pillar of the economy that is struggling to recover after years of political turmoil, and would also undermine Sisi's assertions that Cairo has brought under control Sinai Province's insurgency.

Sinai Province has killed hundreds of Egyptian soldiers and police since Sisi, as army chief, toppled Islamist President Mohamed Mursi in 2013 after mass protests against his rule.

Sisi was elected president last year on promises he would stabilise Egypt and rebuild its shattered economy. Critics say his tough crackdown on Islamists will only create more radicals in Egypt, which has fought militants for decades.

REVENGE FOR RUSSIAN AIRSTRIKES?

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.

The ministry said the second black box, which contains the cockpit voice recorder (CVR), was partially damaged and much work was required to extract data from it.

Sinai Province has said it had brought down the airliner "in response to Russian air strikes that killed hundreds of Muslims on Syrian land".

In an audio message posted on a Twitter (Xetra: A1W6XZ - news) account used by the group, Islamic State's Egyptian affiliate insisted on Wednesday it was behind the crash. The claim could not immediately be authenticated.

"We, with God's grace, are the ones who brought it down, and we are not obliged to disclose the mechanism of its demise," the speaker said.

Russia, an ally of Syrian President Bashar al-Assad, launched air raids against opposition groups in Syria including Islamic State on Sept. 30. The hardline group has called for war against both Russia and the United States in response to their air strikes in Syria.

Late on Wednesday the Irish Aviation Authority (IAA) directed all Irish airlines on Wednesday not to fly to or from the Sinai Peninsula until further notice. The Russian-operated plane was registered in Ireland (Other OTC: IRLD - news) and the IAA is taking part in the official investigation into the crash. (Additional reporting by Tim Hepher and John Irish in Paris, Doina Chiacu and Susan Heavey in Washington, Lin Noueihed in Cairo and Alexander Winning in Moscow; Writing by Michael Georgy; Editing by Alison Williams and Gareth Jones)