Belgian police have arrested seven people in raids in their investigation into Islamic State suicide bombings in Brussels.

The federal prosecutor’s office said six persons were held during 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. Public broadcaster RTBF said a seventh man was arrested in the Forest borough of Brussels early on Friday.

Islamic State suicide bombers hit Brussels airport and a metro train on Tuesday, killing at least 31 people and wounding some 270 in the worst such attack in Belgian history.

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The daily De Standaard said on Friday police had arrested a man who was filmed by security cameras in the airport terminal next to two bombers who blew themselves up there. Prosecutors did not confirm the arrest and it was not known if the man was among the seven detained overnight.

The attack in Brussels, home to the European Union and NATO, has heightened security concerns around the world and raised questions about EU states’ ability to respond in an effective, co-ordinated way to the Islamist militant threat.

US Secretary of State John Kerry arrived in Brussels on Friday for talks with Belgian and European Commission leaders to offer US assistance in security cooperation against terrorism.

The Islamic State militant group also took credit for co-ordinated attacks in Paris in November that killed 130 people at cafes, a sports stadium and concert hall.

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. The news website Politico Europe said investigators had only questioned Abdeslam for a single hour in the four days between his arrest on March 19 and the Brussels bombings.

Belgian daily De Morgen said investigators had identified a new suspect they believe played a role in the Brussels bombings, naming him as 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 along with Mohamed Abrini, Najim Laachraoui and Khalid El Bakraoui. Hamed was also suspected of involvement in the Paris attacks, De Morgen said.

One man was killed in a shootout with police on March 15 that led to the discovery of assault weapons and explosives and the arrest of Abdeslam, 26, and another suspect on March 18.

Belgium on Thursday lowered its security alert level one notch to three from the highest level, four, but officials did not say what that would mean in terms of security measures that have included a heavy police and military presence in Brussels.

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