Belgian authorities are hunting a cousin of the El Bakraoui brothers who blew themselves up in the Brussels airport and metro attacks, a source close to the inquiry said Wednesday.

Oussama Atar, who spent years in jail in Iraq before returning to Belgium in 2012, is wanted on suspicion of involvement in the March 22 Islamic State-claimed bombings, the source told AFP on condition of anonymity, confirming Belgian media reports.

Police arrested Atar's mother, sister and one of his friends during raids in Brussels on August 11 but released them shortly afterwards, said the source who was not authorised to speak on the record.

Investigators have "very strong" suspicions that Belgian-Moroccan national Atar was linked to the attacks in which his cousins Ibrahim and Khalid El Bakraoui blew themselves up with a third bomber, killing 32 people, the source said.

Atar was one of the "most wanted in Belgium and even in Europe", the source added.

Khalid El Bakraoui attacked Maalbeek metro station while his brother blew himself up at Brussels' Zavantem Airport. Both were part of a cell with links to the November attacks on Paris.

Atar's brother Yassine was arrested with two other men five days after the Brussels attacks and remains in detention.

Two of his other cousins, Moustapha and Jawad Benhattal, were arrested on June 18 on suspicion of planning an attack during a public screening of a Euro 2016 football match between Belgium and Ireland, Belgian media reported.

Atar was arrested in Ramadi, Iraq in February 2005 and sentenced to 10 years in jail for illegally crossing the border from Syria, his lawyer at the time, Vincent Lurquin, told AFP.

After spending time in jails, including the notorious Abu Ghraib prison, he was freed and sent back to Belgium in September 2012.

"I don't know what happened to him after that," Lurquin said.

Atar said in an interview in 2011 that he had gone to Syria to study Arabic then went to Iraq to take medicines to people in need. His case had been backed by rights group Amnesty International.