Seven terror suspects were in custody in Belgium and France on Friday as under-fire European authorities stepped up the fight against jihadist networks following triple bombings in Brussels claimed by the Islamic State group.
Six people were being held after raids across the Belgian capital Thursday, two days after airport and metro suicide blasts that left 31 people dead and 300 injured.
And in the Paris suburbs, police arrested a man accused of plotting an attack in France that was "in the advanced stages" and found a small stash of explosives.
In Brussels, families faced an agonising wait after forensic experts warned it could take weeks to identify fatalities.
US Secretary of State John Kerry arrived for talks with the Belgian authorities, who have faced heavy criticism over how the Brussels attackers -- at least three of whom were known to authorities -- slipped through the net.
The man arrested in Paris was a French national who "belongs to a terrorist network that sought to strike our country", French Interior Minister Bernard Cazeneuve said late Thursday, hailing the "major arrest".
Police evacuated an apartment block in the rundown northern suburb of Argenteuil, where a small quantity of explosives was found.
While Cazeneuve said no link to the Paris or Brussels attacks had emerged, police sources said Friday that the suspect had in July been found guilty in absentia, alongside Paris ringleader Abdelhamid Abaaoud, of being part of a group planning to go to Syria.
Named by police sources as Reda K., Cazeneuve said the suspect had been under surveillance "for several weeks" and his arrest was the result of "close and constant cooperation between European services".
European authorities are under huge pressure to better coordinate the tracking of homegrown extremists and those returning from Syria, as evidence grows of a thriving jihadist network straddling France and Belgium.
Prosecutors have confirmed that Khalid El Bakraoui, who blew himself up at Maalbeek metro station shortly after his brother Ibrahim did the same at Zaventem airport, was the subject of an international terrorism warrant over the Paris attacks.
Ibrahim El Bakraoui had been arrested and deported by Turkey, which had warned Belgium he was a "foreign terrorist fighter," Turkish President Recep Tayyip Erdogan said Wednesday.
The brothers were also listed in American terrorism databases, television network NBC reported.
EU Home Affairs Commissioner Dimitris Avramopoulos said Thursday the "attacks did not come as a surprise", raising further questions why international authorities failed to stop the bombers.
Belgium's interior and justice ministers, Jan Jambon and Koen Geens, offered to quit Thursday after widespread criticism over intelligence failures, but Belgian Prime Minister Charles Michel refused to accept their resignations.
The Brussels attacks also came four days after Salah Abdeslam, the prime suspect in the Paris attacks who had been on the run for four months, was arrested in a dramatic raid in the city, just around the corner from his family home.
Abdeslam's lawyer Sven Mary said Thursday his client "didn't know" in advance about the Brussels attacks, and said he would no longer fight extradition to France.
A huge manhunt is ongoing for a third attacker at Brussels airport whose bomb did not go off, a man wearing a hat seen on security footage.
Police are also hunting a man with a large bag captured on CCTV talking to Khalid El Bakraoui at Maalbeek station, who then did not get on to the train with the bomber, police sources told AFP.
A series of raids in the capital Thursday yielded six arrests -- included three people detained "outside the door of the federal prosecutor's office", a spokesman for the prosecution service said.
It was unknown whether the arrests included either of the key suspects sought over the worst attacks in the country's history.
Belgium has lowered its terror alert to the second-highest level for the first time since the attacks, but the police and military presence on the streets of the capital remains high.
People of around 40 nationalities were killed or wounded in the attack, testament to the cosmopolitan nature of Europe's symbolic capital.
Very few of the dead have been formally identified. Forensic experts, working with teeth, fingerprints and DNA, are sometimes relying just on tiny fragments of bodies and warn that the process could take weeks.
Tales of lucky escapes from the attacks have been emerging, along with cases of tragic ill fortune of people caught in the wrong place at the wrong time.
Among only three fatalities formally named was Peruvian Adelma Marina Tapia Ruiz, 37. Her husband Christophe Delcambe, and their three-year-old twin daughters, only survived because the girls had run off and their father had chased after them.
Harrowing new footage of the moments after the airport attack has emerged on Belgian television, showing a lone baby left crying in the wreckage next to the lifeless body of a woman.