Belgium on Monday freed the sole suspect charged over last week's Islamic State attacks in Brussels that left 35 dead, raising fresh questions about the handling of the case by under-fire Belgian authorities.

Prosecutors charged Faycal C. on Saturday with "terrorist murder" and were investigating whether he was a third airport attacker who fled after his bomb did not go off, but said there was now a lack of evidence.

His release comes as a new blow to an inquiry dogged by accusations that Belgium missed a series of leads in cracking down on a jihadist network linked to the Brussels bombings as well as the November Paris attacks that killed 130 people.

Police earlier Monday released new CCTV footage of a third suspect in the March 22 Zaventem airport attack, the so-called "man in the hat" seen with the two suicide bombers.

Mourners meanwhile held emotional Easter Monday prayers at a medieval cathedral in central Brussels in memory of the 35 people killed and 340 injured in Belgium's worst ever terror attacks.

Grieving airport personnel and members of the emergency services carried trays of full of candles at the cathedral of Saints-Michel-et-Gudule, and were applauded by the congregation.

"No violence in the name of God can be tolerated," the Roman Catholic Archbishop of Malines-Brussels, Jozef de Kesel, told worshippers.

"Peace is more than just the absence of violence. There cannot be a real life together without a profound and sincere respect for others."

- 'Not substantiated' -

Belgian officials said Monday that the death toll had climbed to 35 after four people died in hospital.

The dead include four Americans as well as people from countries from China to Britain, Sweden to Peru, testament to the cosmopolitan nature of a city that is home to both the European Union and NATO.

Ninety-six remain in hospital.

Brussels is still trying to get back on its feet, with the airport saying it would carry out a test run Tuesday to see if repair work in the wrecked departure hall was satisfactory, but it could not give a firm date for resuming services.

Belgian authorities are continuing to face criticism over whether they could have prevented the tragedy, as the links to the Paris attacks become clearer by the day.

The Belgian federal prosecutor's office said in a statement that "the indications that led to the arrest of Faycal C. were not substantiated by the ongoing inquiry. As a result, the subject has been released by the examining magistrate."

A source close to the inquiry told AFP: "Investigators have established that he was not the 'man in the hat'."

Belgian media had identified the man as Faycal Cheffou, who claimed to be a freelance journalist.

With the manhunt still underway, police released fresh video of a man in a hat and white jacket pushing a trolley with a large bag through the departure hall, next to suicide bombers Ibrahim El Bakraoui and Najim Laachraoui.

- European arrests -

Bomb-maker Laachraoui's DNA was found on some of the explosives used in Paris.

Ibrahim El Bakraoui's brother Khalid, who blew himself up on a Brussels metro train shortly after the airport blasts, is meanwhile believed to have rented a property linked to Paris prime suspect Salah Abdeslam.

Abdeslam was arrested in Brussels on March 18 just metres from his family home after four months on the run.

And Turkey accused Belgium last week of ignoring a clear and present danger after revealing it had deported Ibrahim El Bakraoui as a "terrorist fighter" last year after arresting him near the Syrian border.

Two Belgian ministers offered to resign after the Turkish link emerged.

Prosecutors earlier said three men arrested in raids in Belgium at the weekend had been charged with "participation in the activities of a terrorist group" but that no direct link had been established with the Brussels attacks.

In the latest piece in the puzzle of the jihadist cross-border networks, police arrested a 32-year-old Frenchman in Rotterdam Sunday on suspicion of planning a terror attack, Dutch prosecutors said.

The man is thought to have been planning an attack in France in the name of the Islamic State group along with Reda Kriket, who was detained near Paris on Thursday, a French police source told AFP.

Belgian prosecutors at the weekend also charged two men with involvement in the Kriket plot, including one shot in the leg after a dramatic stand-off at a tram stop in Brussels on Friday.

An Algerian held in Italy as part of a probe into fake ID documents used by the Paris and Brussels attackers is still being interrogated but refused to answer questions, a judicial source said.