Police have raided Brussels neighbourhoods in an operation the mayor said was linked to both the airport and subway bombings and to the arrest in the Paris suburbs of a man who may have been plotting a new attack in France.
Three people were detained, with two of them shot in the leg, the federal prosecutor's office said. The operation was conducted in the Schaerbeek district, which was raided on Thursday night, as well as the neighbourhoods of Forest and Saint-Gilles, it added.
Belgium's state broadcaster said one person was carrying a bag of explosive material. The area remained cordoned off, even after heavily armed officers had left, a resident said.
Three of six people detained on Thursday in the investigation have been released, prosecutors said.
The broadcaster, RTBF, quoted Schaerbeek district mayor Bernard Clerfayt as saying the raid was linked to the Brussels attacks and Thursday's arrest of a man in France.
On the third and final day of national mourning for Tuesday's attacks that killed 31 people, Belgium's Prime Minister Charles Michel missed a wreath-laying ceremony at the airport with US Secretary of State John Kerry because of the police raids.
Mr Kerry, in a hastily arranged visit, defended Belgium's counter-terrorism efforts despite a series of security and intelligence failings in the run-up to the bombings on Tuesday.
Confirming that several FBI agents are involved in the investigation, Mr Kerry said the "carping" about Belgium's shortcomings "is a little bit frantic and inappropriate".
Top members of Belgium's embattled government are facing criticism over its counter-terrorism efforts since - and before - the November 13 Paris attacks that killed 130 people and that authorities believe were plotted from Belgium.
Mr Kerry also said the US and other countries had already scheduled meetings with Belgium prior to the attacks about improvements they could make to their laws, intelligence collection and attempts to blunt the radicalisation of youth.
The Islamic State group has claimed responsibility for the attacks in Paris and Brussels, and Mr Kerry lashed out at the extremist group.
"We will not be deterred," he said. "We will come back with greater resolve – with greater strength – and we will not rest until we have eliminated your nihilistic beliefs and cowardice from the face of the Earth."
Authorities also announced that American, British, Chinese, French and Dutch citizens were among the dead.
A manhunt has been under way since Tuesday for one of the Brussels airport attackers who was recorded on a surveillance video and fled the scene. Prosecutors have not said how many attackers there were in total, or how many accomplices might be at large.
Belgian prosecutors said raids on Thursday night targeted central Brussels, Jette and the Schaerbeek neighbourhood, where police had earlier found a huge stash of explosives and bomb-making material in an apartment used by the Brussels attackers.
French counter-terrorism police also detained a man on Thursday who officials say was in the advanced stages of an attack plot.
Officials told The Associated Press that the suspect Reda Kriket (34), had a past Belgian terrorism conviction and was linked to the suspected ringleader of the Paris attacks, Abdelhamid Abaaoud.
Elsewhere, Belgium's nuclear agency said it has withdrawn the entry badges of some staff and denied access to other people recently amid concern the country's nuclear plants could be a target for extremists.
The move at some plants "is not necessarily linked with the terrorist attacks," said nuclear control agency spokeswoman Nele Scheerlinck, noting the decision to deny access usually takes weeks.
Immediately after Tuesday's attacks, security was boosted around the country's nuclear sites and hundreds of staff were sent home.
Last month, authorities said searches in the wake of the Paris attacks uncovered video linked to a person working in Belgium's nuclear industry.
Belgian media reported this week that Brussels attackers Ibrahim and Khalid El Bakraoui had video recordings of the home of a senior official at the Mol nuclear waste facility in the Flanders region.
