Belgian police have arrested six people in their probe of the Islamic State suicide bombings in Brussels, while authorities in France said they had thwarted an "advanced" militant plot there.
The federal prosecutor's office in Belgium said the arrests came during police searches in the Brussels neighbourhoods of Schaerbeek in the north and Jette in the west, as well as in the centre of the Belgian capital.
The arrests came days after suicide bombers hit the Brussels airport and a metro train, killing at least 31 people and wounding some 270 in the worst such attack in Belgian history.
The Islamic State militant group has claimed responsibility for the bombings, as well as coordinated attacks in Paris in November that killed 130 people.
In Paris on Thursday, authorities arrested a French national suspected of belonging to a militant network planning an attack in France.
French Interior Minister Bernard Cazeneuve said the arrest helped "foil a plot in France that was at an advanced stage".
Earlier on Thursday (local time), Belgium's interior and justice ministers offered to resign over a failure to track an Islamic State militant expelled by Turkey as a suspected fighter and who blew himself up at Brussels Airport.
Brahim El Bakraoui was one of three identified suspected suicide bombers who hit the airport and metro train. 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, killed 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.
Interior Minister Jan Jambon and Justice Minister Koen Geens tendered their resignations to Prime Minister Charles Michel, who asked them to stay on.
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.
Security sources told Belgian media the other suicide bomber at the airport was Najim Laachraoui, a veteran Belgian Islamist fighter in Syria suspected of making explosive belts for November's Paris attacks.
Laachraoui's younger brother Mourad issued a statement condemning his actions, in the first public reaction from a family member of one of the Brussels attackers.
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".