Scores of people remain in intensive care following suicide Tuesday's bomb attacks in Brussels that left 31 people dead, Belgium's health minister says.

Maggie de Block said that of about 300 wounded people, 61 were still in a critical condition, and suggested that the death toll could rise further.

Earlier, prosecutors confirmed they had identified two of the four attackers as brothers Khalid and Brahim el-Bakraoui.

Two other attackers have yet to be named. One died, another is on the run.

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Brahim el-Bakraoui blew himself up in the attack at Zaventem airport that killed 11 people while Khalid struck at Maelbeek metro, where 20 people died, prosecutors said.

Unconfirmed reports say another of the attackers was the wanted jihadist Najim Laachraoui, who is thought to have links with last year's attacks in Paris.

So-called Islamic State (IS) has said it was behind the attacks.

EU interior and justice ministers are due to hold a crisis meeting in Brussels on Thursday to discuss their response to the bombings.

The man on the left is believed to have died at the airport. The man on the right, wearing the hat, is thought to have fled the scene.

Van Leeuw said the man in the hat had left a bag containing "the biggest bomb", which later partially exploded after police had evacuated the terminal, injuring no-one.

Reports in Belgian and French media suggest the man on the left is Najim Laachraoui, but this is not confirmed. Analysts say Laachraoui is believed to be a key bomb maker, and French media say he played a key role in the terror attacks in Paris.

Prosecutors said brothers Ibrahim and Khalid El Bakraoui had carried out attacks at Zaventem airport and Maalbeek metro station, while bomb-making expert Najim Laachraoui was identified by police sources as the second airport bomber. Photo: AFP.

 Van Leeuw said a taxi driver had told police he had picked up the three men from an address in the Schaerbeek area of Brussels.

The apartment was raided later on Tuesday and bomb-making materials, including 15kg (33lb) of high explosive, were found.

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A note from Brahim el-Bakraoui was found in a nearby rubbish bin. In it, he wrote: "I'm in a hurry (...) they're looking for me everywhere. I'm not safe any more. If I give myself up they'll put me in a cell.

Van Leeuw said the brothers, who were Belgian nationals, were known to police and had criminal records. They were identified by DNA records.

Broadcaster RTBF quoted a police source saying that Khalid el-Bakraoui had used a false name to rent a flat in the Forest area of Brussels where police killed a gunman in a shootout last week.

It was during that raid that detectives found a fingerprint of Salah Abdeslam, the main suspect in the Paris terror attacks of 13 November.

He was arrested in a raid in Brussels last Friday.

Turkey said on Wednesday that Brahim was detained by Turkish officials on the border with Syria in June 2015 and deported to the Netherlands.

Belgium has declared three days of mourning. Photo: AFP.

President Recep Tayyip Erdogan said Belgium had "ignored our warning that this person is a foreign fighter".

Belgian Justice Minister Koen Geens said he was aware that the suspect had been deported from Turkey but denied that he had been flagged as a possible terrorist.

Belgium has raised its terrorism alert to the highest level, and its international airport will remain closed on Thursday.