US intelligence: A bomb planted by ISIS most likely brought down the crashed Russian plane

US intelligence reportedly points to a bomb as the most likely culprit of the plane crash in Egypt last week, US and European security sources said on Wednesday.

The Russian-operated Airbus A321M crashed on Saturday shortly after taking off from the resort on its way to St Petersburg, killing all 224 people on board.

The bomb was likely planted by the terrorist group ISIS or an ISIS affiliate, CNN reported.

The intelligence is reportedly based partly on monitoring of internal ISIS messages.

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.

Investigations are ongoing, however, and US and European officials haven't made any formal conclusions yet.

"There is a definite feeling it was an explosive device planted in luggage or somewhere on the plane," an unnamed official told CNN.

Officials are looking into whether ground crews, baggage handlers, or other ground staff could have planted a bomb on the plane, according to NBC News. A review of the passenger manifest and crew list didn't reveal anyone with ties to terrorism.

A White House spokesman didn't immediately respond to a request for comment on the report.

Separately, Britain said on Wednesday that the plane might have been brought down by an explosive device. The aircraft crashed in Egypt after taking off from Red Sea resort of Sharm el-Sheikh.

"We have concluded that there is a significant possibility that the crash was caused by an explosive device on board the aircraft," Britain's foreign secretary, Philip Hammond, said after a meeting of the government's crisis response committee chaired by Prime Minister David Cameron.

As a precautionary measure, it said, the government had decided that flights due to leave the resort for Britain on Wednesday evening would be delayed to allow time for a team of UK aviation experts, currently on their way, to make an assessment of the security arrangements in place at the airport.

REUTERS/Peter KovalevA relative of victims of a Russian airliner, which crashed in Egypt, mourns at a hotel near Pulkovo International Airport in St. Petersburg, Russia, October 31.

That assessment was expected to be completed later and there were no more flights due to take off from Britain to Sharm el-Sheikh on Wednesday.

"We would underline that this is a precautionary step and we are working closely with the airlines on this approach," the statement said.

Egypt, a close ally of the United States and the most populous Arab country, dismissed a similar claim of responsibility for the crash by Islamic State on Saturday.

"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 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."

Remarks earlier on Wednesday by Britain's Cameron, who was due to hold talks in London with Sisi on Thursday, of concerns "the plane may well have been brought down by an explosive device" drew criticism from Egyptian Foreign Minister Sameh Shoukry.

"SOMETHING STOWED" ON BOARD

REUTERS/StringerEgypt's Prime Minister Sherif Ismail (R) listens to rescue workers as he looks at the remains of a Russian airliner after it crashed in central Sinai near El Arish city, north Egypt, October 31, 2015.

A Russian aviation official said 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.

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.

He told CNN he was "somewhat surprised" by the British statement.

"This is a matter for the investigation to clarify and we should not prejudge or take any measures that might have implications," Shoukry said. "Implication also that the fact that a very large number of Egyptians who rely heavily on the tourist industry."