BRUSSELS (REUTERS) - Belgian police arrested three more people on Friday as investigations into Tuesday's suicide bombings by Islamist militants in Brussels threw up more links to killings in Paris last year.

The federal prosecutor's office said the operation was connected to the arrest in Paris on Thursday of an Islamist convicted in Belgium last year and suspected of plotting a new attack.

Nine people in total have been arrested since Thursday in Belgium and two in Germany, as European authorities swoop on Islamic State in Iraq and Syria (ISIS) militants they link both to the Brussels bombings that killed 31 people and to the attacks in Paris last November that killed 130.

Ahead of one of the arrests, heavily armed police and troops with trucks cordoned off an area around a major intersection in the northern Brussels borough of Schaerbeek. Three blasts could be heard, which the local mayor Bernard Clerfayt said were controlled explosions.

Belgian public broadcaster RTBF quoted Clerfayt as saying the suspect had been detained after being wounded and that he was linked to Tuesday's suicide bombings in Brussels.

It initially said he had been found to be in possession of a suitcase full of explosives but later news reports did not confirm that explosives were found.

Witnesses said police appeared to shoot the man in the leg at a tram stop and that he appeared to have his daughter with him.

"We heard 'Don't move'. The man was sitting at the bus stop, a bus stop with a glass wall, and we heard a small detonation and a big detonation," said Norman Kabir, 38, an electrician who lives and works nearby.

"Then the police came, took the little girl who was shouting 'Dad', she seemed terrified and the man got shot in the leg anyway because he was still moving," Kabir told reporters.

"Then the police asked him to move his bag. He was lying on the ground, but he did it and pushed the bag and a robot from the mine-sweeping brigade arrived. It came, grabbed the bag and took it away, then they took the guy, put him in a car and left. It took 20 seconds."

Video showed the man lying on his side, shattered glass from the tram shelter smashed by bullets at his feet.

ISIS suicide bombers hit Brussels airport and a metro train on Tuesday in the worst such assaults in Belgian history. Investigators believe they were carried out by the same cell behind November's gun and bomb attacks in Paris.

MORE ARRESTS

The Belgian federal prosecutor's office said six people were held in Brussels on Thursday, of whom three were released and three were remanded in custody facing possible charges.

Three others were detained on Friday following the arrest overnight in France of Reda Kriket, a 34-year-old Frenchman sentenced to 10 years in absentia in Brussels last July as part of an Islamist recruiting network dubbed the Syrian Connection.

In the same case, Abdelhamid Abaaoud, the suspected ringleader of the Paris attacks later killed by French police, got 20 years, also in absentia, while Khalid Zerkani, a Moroccan recruiter of militants in Europe, was jailed for 12 years.

According to a report in Le Point magazine, Zerkani was a mentor to the two other men and to Najim Laachraoui, one of the Brussels airport bombers who is also suspected of having made explosive belts for the Paris attacks.

In Germany, Der Spiegel magazine said German police had arrested two people, one of whom had received phone messages with the name of the metro station bomber and the word "fin" - French for "end" - three minutes before the metro blast. The German interior ministry declined comment.

A person familiar with the investigation in Belgium said one of the people arrested there was believed to be a suspected accomplice caught on security camera footage with the metro station bomber.

"We have strong indications that this is the suspect who was hunted for the last couple of days. The identification is still ongoing," the source said.

However he said those arrested before midday on Friday did not include a third suspect seen on video alongside the two who blew themselves up at the airport.

The attacks in Brussels, home to the European Union and Nato, have heightened security concerns around the world and raised questions about EU states' ability to respond in an effective, coordinated way to the Islamist militant threat.

Belgium's interior minister Jan Jambon on Friday blamed police negligence. "Someone in our police apparatus blundered," he told a special session of parliament.

Jambon offered to resign on Thursday over a failure to track one of the airport bombers, Brahim El Bakraoui, 29, who had been expelled last year by Turkey as a suspected fighter. Bakraoui's brother Khalid, 26, was the bomber who struck the metro train.

"JE SUIS BRUXELLOIS, IK BEN BRUSSEL"

US Secretary of State John Kerry arrived in Brussels on Friday to offer US assistance in security. Two of the Brussels victims were Americans. "Je suis bruxellois. Ik ben Brussel," Kerry said in French and Dutch, expressing solidarity with the people of Brussels in the two languages of the Belgian capital.

In Paris on Thursday, authorities arrested Kriket, a French national suspected of belonging to a militant network planning an attack in France. French officials did not directly tie that plot to the Brussels attacks, but Clerfayt, the Schaerbeek mayor, said the man wounded on Friday was linked to both investigations.

French President Francois Hollande said the hunt for Islamist militant cells would continue. "We know there are other networks. Even if the one behind the attacks in Paris and Brussels is in the process of being wiped out, a threat is still there," he said.

The Brussels attacks came a week after Belgian police killed a militant during a house raid that led them to Salah Abdeslam, the only suspected participant in the Paris attacks to have been captured alive.

His lawyer says he has been cooperating with the investigation but did not know of the plan to attack Brussels.

However, on Friday prosecutors said Abdeslam had declined to talk about Tuesday's attacks, having declared soon after his capture that he would exercise his right to be silent.

Belgian daily De Morgen said investigators had identified a new suspect they believe played a role in the Brussels bombings, 28-year-old Syrian Naim al-Hamed. The paper said he was on a list circulated to the security services of other European countries after Tuesday's attacks, and was also suspected of involvement in the Paris attacks.

ISIS posted a video on social media calling the Brussels blasts a victory and featuring the training of Belgian militants suspected in the Paris attacks.