The UK government has confirmed concerns a bomb “may well have” brought down a Russian passenger jet in Egypt, while a US intelligence official said it was a “definite feeling”.

Investigations were continuing into the crash that killed all 224 people onboard when an unnamed US intelligence official made the comments on Thursday morning.

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

Just one body has been released to a Russian family for burial. There are 33 others in the process of formal identification. Photo: Getty

The plane was en-route from an Egyptian resort area to St Petersburg on Saturday when it suddenly dropped more than 1000m before crashing in Egypt’s Sinai peninsula.

According to the US official, it was most likely planted by ISIS or an affiliated group, although no formal conclusion had been reached.

US spies were reportedly monitoring messages associated with the terror groups in the lead-up to the crash, and following it, which had led them to believe the extremist group was involved.

The official told CNN “there had been additional activity in Sinai that had caught our attention” prior to the crash.

Earlier on Thursday, Prime Minister David Cameron’s office confirmed concerns were growing an explosive device may have been onboard the plane before it left the airport.

“While the investigation is still ongoing we cannot say categorically why the Russian jet crashed,” the PMO 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.”

Earlier this week, the Egyptian government said it could not have been shot down at the cruising altitude of 32,000ft.

The Saint Petersburg-bound plane operated by Russian airline Kogalymavia crashed 23 minutes after taking off from the Red Sea resort of Sharm el-Sheikh, killing all onboard, most of whom were Russian tourists.

Kogalymavia insisted the plane crashed due to “external action”, not technical fault or human error.

The plane’s two black boxes were recovered. An examination of their recordings began on Tuesday in Egypt. It could take days or weeks to finalise.

But Egypt’s president warned this week that it could take years to definitively establish what caused the crash.

IS affiliate reaffirms responsibility claim

Islamic State-affiliated militants claimed responsibility for the crash soon after, although the claim was initially dismissed by the Egyptian and Russian governments.

President Abdel Fattah al-Sisi’s government is locked in a civil war with militant Islamist groups, which seem to be aligned with Islamic State. An admission that terrorists were able to sneak aboard the plane could be seen as weakness.

al-Sisi reasoned that the IS group did not have access to the technology required to bring the plane down at a high altitude.

But on Wednesday, the IS group insisted it was responsible for the crash and said “we will disclose the mechanism of its demise at the time that we want”.

“So go to the wreckage, search, bring your black boxes and analyse, give us the summary of your research and the product of your expertise and prove that we did not bring it down or how it came down,” the speaker said in Arabic.

The Sinai peninsula, where the plane crashed, has been disputed territory for the past year following an insurgency of Islamic State-affiliated militants.

The UK government delayed flights due to leave Sharm el-Sheikh on Wednesday evening to allow time for UK aviation experts, travelling to the site, to assess security at the airport.

-with AAP