Emerging as a key central figure in the terrorist super-cell behind the attacks in Brussels and Paris is a pot-bellied militant recruiter who from the sidewalks of Molenbeek convinced young men to bring mayhem to Europe.

The bearded and balding Khalid Zerkani, 42, is behind bars in Belgium appealing a 12-year sentence but his influence was crucial to a vast terror network that would go on to kill more than 160 people in the biggest ever terror attacks on either French or Belgian soil.

"Zerkani perverted a whole generation, particularly in the Maritime neighbourhood of Molenbeek," prosecutor Bernard Michel told a courtroom in February, when Zerkani and 31 others were sentenced for their ties to terrorism inspired by the Daesh group.

"He was active in recruiting, logistics, financial help and even in prison, he encouraged other inmates to take up the jihad banner," Michel said.

Only 13 of defendants were actually present in court in February, with the rest either presumed killed in Syria or more worrying, whereabouts unknown.

In an embarrassment for the Belgian authorities, several of these young men are no longer missing, but actively being hunted by police, having returned from Syria unnoticed to carry out attacks in Europe.

Zerkani was born in Zinata, Morocco in 1973 and was arrested by police in 2014 after several years of defying authorities with his off-the-radar recruitment of foreign fighters for Daesh in Syria in gritty Molenbeek.

Belgium is the biggest supplier of foreign Daesh fighters to Syria per capita and Zerkani, until his arrest, was their biggest recruiter, authorities now believe.

Thirty other Zerkani-linked suspects are separately on trial in Brussels, many also with their whereabouts unknown.

Until Tuesday's deadly attacks in Brussels, by far the biggest name on the trial list was Paris attacks ringleader Abdelhamid Abbaoud, who was killed by French police in November.

But now police are taking another look at the Zerkani suspects after suicide-attacker and presumed Paris bomb-maker Najim Laachraoui, 24, blew himself up in Brussels airport on Tuesday.

Also on the list is French national Reda Kriket, 34, who was arrested in a Paris suburb on Thursday, as is Gelel Attar of Molenbeek who was arrested in Morocco in January and was a close friend of Abbaoud and Chakib Akrouh, another of the Paris attackers.

The "emir"

According to people that knew him, Zerkani had a powerful influence on his young recruits, with an undeniable aura that was not discernable to outsiders.

"He's the emir, the leader... He organized meetings in attics several times of week for candidates seeking to leave for Syria" that often included children still toting their school bags, a suspect identified as Yacine E. told investigators in a deposition leaked to Belgian daily La Derniere Heure.

"For Zerkani, Islam requires jihad and armed jihad is the highest form that exists," he added.

But with his long beard and unkept demeanor, he was also nicknamed the militant "Father Christmas" on the streets of Molenbeek and described by one legal source as "smelly" and unimpressive.

Zerkani, also known under the nom de guerre Abu Riad, is still having a pervasive effect on the terror battleground in Europe.

His recruit Kriket, the man arrested on Thursday near Paris, received a 10-year sentence in absentia in the Zerkani trial.

French police said they found explosives in the operation and that Kriket was planning an imminent attack.