Najim Laachraoui, one of the Brussels airport bombers, has been identified by several Frenchmen held hostage by Islamic State militants in Syria as one of their former prison guards, sources close to the investigation say.

According to one of the sources, four French journalists previously kidnapped and held in Syria from 2013 to 2014 had identified a guard known as "Abou Idriss".

One of the journalists, Nicolas Henin, "has formally identified" Abou Idriss as being Najim Laachraoui, his lawyer Marie-Laure Ingouf said, confirming reports in French newspapers.

"I can confirm that he was the jailer of my clients," Ms Ingouf said.

Belgian prosecutors have said Laachraoui travelled to Syria in February 2013 to join up with IS forces.

There was no further trace of the Belgian national until he was registered under a false name at the border between Austria and Hungary in September 2015.

Laachraoui, 24, was one of the two suicide bombers who struck Brussels airport on March 22, while a third attacker blew himself up at on a metro train, with the two attacks killing 32 people.

Prosecutors have also linked him to November's attacks in Paris in which 130 people died, saying his DNA was found on a suicide vest and a piece of cloth at the Bataclan concert hall where 90 people were killed.

Police found his DNA on explosives used at the Stade de France, leading investigators to believe he was the bomb maker in both the French and Belgian attacks.

Islamic State militants have also identified Laachraoui as Abou Idriss in the latest edition of its magazine, Dabiq, saying he "prepared the explosives for the two raids in Paris and Brussels".

The former French hostages have already identified two Frenchmen as being among their jailers when they were held in Syria.

One, Mehdi Nemmouche, is in custody accused of killing four people in an attack on the Jewish museum in Brussels in 2014.

The other is Salim Benghalem, who has been sentenced in absentia in France for recruiting for IS and is listed as a "foreign terrorist combatant" by the United States.

AFP/Reuters