Younes Abaaoud
Younes Abaaoud, a 15-year-old from Belgium, is believed to be one of the youngest Islamic State fighters Twitter: @ShirazMaher

France could be facing a new threat less than a month after the deadly Paris attacks as alleged members of the Islamic State (Isis) have claimed ringleader Abdelhamid Abaaoud's younger brother Younes could be "on his way back" for revenge.

Belgian jihadist Abdelhamid was killed on 18 November during a raid conducted by French authorities on an apartment in the Parisian suburb of Saint-Denis after the string of attacks on 13 November left 130 people dead.

Another Belgian jihadist, who claims to be in Syria, posted a Facebook message below a photo of Younes Abaaoud, 15, who in August last year was believed to have joined his older brother to fight for IS in Syria.

"Abu Omar al-Soussi (Abdelhamid Abaaoud's nom de guerre) died as a martyr, but his brother is still alive. We are on our way back to you, cross worshippers."

Younes was only 13 when his brother Abdelhamid forced him to leave the Belgian suburb of Molenbeek to join the ranks of IS in 2014.

Islamic State's 'youngest foreign fighter'

The youngster's identity was tweeted by Shiraz Maher, an academic and senior fellow at the International Centre for the Study of Radicalisation (ICSR) at King's College, London, who wrote: "One of the youngest foreign fighters we know of Younes Abaaoud, who was 13 when he left Belgium to join Isis."

French media reported Abdelhamid used the pretext of a trip to visit his grandfather in Morocco – and claimed to be done with religion – to take Younes to Turkey, via Germany, before crossing into Syria.

A photo of the teenager, was widely circulated, and shows him posing with an AK-47 in his left hand and pointing to the sky with his right forefinger.

In January, their father Omar Abaaoud told Belgian newspaper Het Laatste Nieuws he would "never forgive" Abdelhamid for "brainwashing" his brother, and "bringing shame to our family".

"Our lives are destroyed. Why, in the name of god, would he want to kill innocent Belgians? Our family owes everything to this country," Omar Abaaoud said.

Belgian and French media reports cannot clarify whether the teenager is fighting or living away from the front lines.

Britain has among the highest number of extremists fighting for Isis – at least 500 – but Belgium has the highest proportion based on its population.

French authorities, meanwhile, have stepped up efforts to fight radicalisation with sorties over IS-held territory in Syria following a series of jihadist attacks by gunmen in the country, including the Charlie Hebdo shootings in January. France has been named as a target by IS militants.