Russian investigators say they are now certain a bomb brought down a passenger plane which crashed in Egypt’s Sinai desert last month, killing 224 people.

“One can unequivocally say that it was a terrorist act,” Alexander Bortnikov, the head of Russia’s FSB security service, told a meeting chaired by president Vladimir Putin, according to a transcript published on the Kremlin’s website.

He said traces of explosives had been found in the wreckage of the Airbus A321, which crashed shortly after take off from Egypt’s Sharm el-Sheikh airport and was packed with Russian holidaymakers heading home to St Petersburg.

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Citing experts, Mr Bortnikov said the plane disintegrated in mid-air after the detonation of a bomb with the equivalent of a kilogram of TNT.

Mr Putin has ordered the country’s special services to focus on finding those behind the bombing of the aircraft, calling the attack “one of the bloodiest of crimes”.

“It is not the first time that Russia confronts barbarous terrorist crimes,” Mr Putin said.

“We will search for them anywhere they might hide. We will find them in any part of the world and punish them.”

In a statement on its website, the FSB said it was offering a $50 million reward for “information helping to arrest the criminals” behind the attack.

Mr Putin ordered Russia’s foreign ministry to contact all Moscow’s partners for assistance and said that it was counting on “our friends” to help find and punish those responsible for the plane attack.

Security officials in Egypt told Reuters authorities had detained two employees of Sharm el-Sheikh airport suspected of aiding those who put the bomb on board.

“Seventeen people are being held, two of them are suspected of helping whoever planted the bomb on the plane at Sharm el-Sheikh airport,” one of the officials said.

In response to the findings, Mr Putin pledged to step up air strikes in Syria.

“The combat work of our aviation in Syria must not only be continued, it must be intensified so that the criminals understand that vengeance is inevitable.”

British and US spies had said they had intercepted “chatter” from Islamic State (IS) leaders celebrating the downing of the plane, suggesting that a bomb was possibly hidden in luggage in the hold.

A Sinai-based Egyptian militant group allied to IS has claimed it shot the plane down in response to Russia’s military intervention in Syria in support of president Bashar al-Assad.

In September Russian warplanes took to the skies over Syria in strikes Moscow insisted were aimed at against IS jihadists, targeting weapons depots, ammunition, communications infrastructure and fuel.

The West, however, suspects Moscow’s true aim is to target opponents of Mr Assad, Russia’s ally.

The Airbus A321, operated by Russia’s Kogalymavia airline under the brand name Metrojet, was carrying 200 adult passengers, 17 children and seven crew when it lost contact 23 minutes into its flight.

-with agencies