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

REUTERS/Mohamed Abd El Ghany

An Egyptian military helicopter flies over debris from a Russian airliner which crashed at the Hassana area in Arish city, north Egypt, November 1, 2015.

US intelligence reportedly points to a bomb as the most likely culprit of the plane crash in Egypt last week, according to CNN.

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

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

Investigations are ongoing, however, and US 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.

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 al-Sheikh.

"While the investigation is still ongoing we cannot say categorically why the Russian jet crashed," Prime Minister David Cameron's office 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."

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.

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

Extra consular staff were being sent to the resort to help British holidaymakers there, it said, while those either there or planning to travel were advised to contact their airline or tour operator.

Britain was not changing the level of its travel advice to the area, it added.

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.