Belgian police have arrested six people in raids as part of their investigation into suicide attacks on Brussels, as questions mounted about possible security lapses.
The federal prosecutor's office in Belgium said on Thursday that the arrests took place in the Brussels neighbourhoods of Schaerbeek and Jette, as well as in the centre of the Belgian capital.
No information about the identities of those arrested was released. Judicial authorities will decide on Friday whether they should be charged, the prosecutor's office said.
Suicide bombers hit the Brussels airport and a metro train on Tuesday, killing 31 people and wounding at least 270 in the worst such attack in Belgian history.
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The Islamic State of Iraq and the Levant (ISIL, also known as ISIS) group, which claimed responsibility for the Brussels bombings, also took credit for coordinated attacks in Paris in November that killed 130 people.
"There's certainly still a feeling of insecurity here when it comes to public transport in particular," said Al Jazeera's Jacky Rowland, reporting from Brussels. "There are police and army outside key [metro] stations, and trains don't stop at all locations."
Addressing reporters at an unscheduled conference in Brussels alongside Prime Minister Charles Michel, US Secretary of State John Kerry said on Friday that ISIL was carrying out attacks in Europe because its "fantasy of a caliphate is collapsing before their eyes".
France 'foils' new plot
In a separate development, French authorities said they thwarted a plot there "that was at an advanced stage".
A French national suspected of belonging to a network planning an attack was arrested in Paris on Thursday morning.
Bernard Cazeneuve, the interior minister, said that the arrest helped "foil a plot in France that was at an advanced stage."
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Cazeneuve said that the man arrested was "suspected of high-level involvement in this plan. He was part of a terrorist network that planned to strike France."
The French counterterrorism agency, DGSI, raided an apartment building in the northern Paris suburb of Argenteuil. French TV station ITele reported that explosives had been found in the man's house.
"At this stage, there is no tangible evidence that links this plot to the attacks in Paris and Brussels," Cazeneuve said.
Earlier on Thursday, Belgium's interior and justice ministers offered to resign over the failure to track an ISIL fighter expelled by Turkey who blew himself up at Brussels airport.
Brahim el-Bakraoui was one of three identified suspected suicide bombers believed to have hit the airport. At least one other man seen with them on airport security cameras is on the run and a fifth suspected bomber filmed in the metro attack may be dead or alive.
Bakraoui's brother Khalid, 26, is suspected of killing about 20 people at Maelbeek metro station in the city centre. De Morgen newspaper said he had violated the terms of his parole in May by maintaining contacts with past criminal associates, but a Belgian magistrate had released him.
Turkish President Tayyip Erdogan said Bakraoui, 29, had been expelled in July after being arrested near the Syrian border and two officials said he had been deported a second time.
Belgian and Dutch authorities had been notified of Turkish suspicions that he was a foreign fighter trying to reach Syria.
At the time, Belgian authorities replied that Bakraoui, who had skipped parole after serving less than half of a nine-year sentence for armed robbery, was a criminal but not a "militant".
Interior Minister Jan Jambon and Justice Minister Koen Geens tendered their resignations to Prime Minister Michel, who asked them to stay on.
"There have been growing criticisms about perceived intelligence failings," said our correspondent. "There is an atmosphere of fear and a certain amount of anger and criticism of authorities in [whether] they did their best in the run-up to the bombings."
Salah Abdeslam connection
Belgian public broadcaster VRT said investigators believed that Paris attacks suspect Salah Abdeslam, arrested last Friday, probably planned a similar shooting and suicide bomb attack in Brussels.
Abdeslam's lawyer said the French national wanted to "explain himself" and would no longer resist extradition to France. His lawyer, Sven Mary, said Abdeslam had not been aware of the plan for the Brussels airport and metro attack that was carried out by men who had shared hideouts with him.
Two sources familiar with the matter said the Bakraoui brothers had been on US government counterterrorism watch lists before the attacks. It was not clear how long they had been known to the authorities.
|Mourad Laachraoui, the brother of a suspected ISIL fighter, at a press conference [Peter Dejong/AP]
Security sources told Belgian media the other suicide bomber at the airport was Najim Laachraoui, a veteran Belgian fighter in Syria suspected of making explosive belts for November's Paris attacks.
Laachraoui's younger brother Mourad - considered one of Belgium's big Taekwondo talents - issued a statement condemning his actions.
Laachraoui, 25, gave no warning sign of being radicalised before leaving for Syria in 2013 and breaking all contact with his family, Mourad told a news conference.
"He was a nice boy, and above all he was clever. That's what I remember of him," Mourad said of his brother, who graduated in electromechanics.
He said the last time he saw him, he looked "normal."