Belgium has lowered its security alert level one notch down from four, the highest level, to three, two days after Brussels airport and metro bombings killed 31 people, the Belgian crisis centre said.

Key points: Belgium's ministers offer to resign over failure to track terrorist Ibrahim Bakraoui

Belgium's ministers offer to resign over failure to track terrorist Ibrahim Bakraoui Twice-deported Bakraoui was in the Netherlands last July

Twice-deported Bakraoui was in the Netherlands last July The Netherlands says Turkey did not follow normal procedures to say who he was

The group said it took the decision after reviewing the situation.

"The threat of an attack is less imminent," said Paul Van Tigchelt, head of the Government agency that assesses threat levels.

Crisis Centre director Alain Lefevre said that more metro stations would gradually open together with police and military guards.

He added that the airport, which will stay closed until at least Monday, would reopen with entry checks and sniffer dogs.

The news came as Belgium's interior and justice ministers offered to resign over the failure to track an Islamic State militant expelled twice by Turkey as a suspected fighter and who blew himself up at Brussels airport.

Ibrahim El Bakraoui was one of three identified suspected suicide bombers who hit the airport and a metro train in the worst terrorist attack in Belgian history.

At least one other man seen with them on airport security cameras is on the run and a fifth suspected bomber filmed in the metro attack may be dead or alive.

Interior Minister Jan Jambon and Justice Minister Koen Geens tendered their resignations to Prime Minister Charles Michel, who asked them to stay on.

"The Prime Minister told me in the current situation, in time of war, you cannot leave the field," said Mr Jambon, a right-wing Flemish nationalist.

The security lapses in a country that is home to the European Union and NATO have drawn international criticism of an apparent reluctance to tackle Islamist radicals effectively.

Turkish officials said Bakraoui, 29, had been expelled last July after being arrested at Gaziantep near the Syrian border and again in August after he returned to Antalya. Belgian and Dutch authorities had been notified of Turkish suspicions that he was a foreign fighter trying to reach Syria.

At the time, Belgian authorities replied that Bakraoui, who had skipped parole after serving less than half of a 9-year sentence for armed robbery, was a criminal but not a militant.

"You can ask how it came about that someone was let out so early and that we missed the chance to seize him when he was in Turkey. I understand the questions," Mr Jambon said.

"In the circumstances, it was right to take political responsibility and I offered my resignation to the Prime Minister."

Interpol release images of Khalid (L) and Ibrahim (R) El Bakraoui - two Belgian brothers identified as suicide bombers. ( AFP: Interpol )

Turkey failed to follow normal procedures: Justice Minister

Meanwhile, the Justice Minister in The Hague said the Netherlands did not realise Bakraoui was a dangerous suspect when he arrived at Amsterdam airport, as Turkey failed to follow normal procedures when expelling him.

The terrorist slipped through the net and into the Netherlands last July, as a result of miscommunication between countries, Ard van der Steur said on Thursday.

"He wasn't registered anywhere and the Turkish authorities didn't give the reason why he was deported," Mr van der Steur said.

"Turkey did not follow normal procedures and ... tell us he was being deported to the Netherlands."

Turkish President Tayyip Erdogan said on Wednesday that Turkey had warned Belgian and Dutch officials that Bakraoui, a convicted armed robber on parole, was a militant.

Officials later added that he had twice been deported — he had been expelled last July after being arrested at Gaziantep near the Syrian border and again in August after he returned to Antalya.

In a written answer to parliament, Mr van der Steur confirmed Bakraoui arrived on July 14 after Dutch diplomats in Ankara were informed he was being expelled and had asked to be flown to Amsterdam. But the Dutch had no evidence that he had arrived twice.

"There were no suspicions against him in the Netherlands. The person concerned was not registered in the relevant investigation system," he wrote.

He said Bakraoui was not registered on alert systems at Schipol airport.

Meanwhile, the two brothers who carried out the suicide attacks were known to US Government agencies before the attacks, sources familiar to the matter told Reuters.

The sources said that Khalid El Bakraoui and Ibrahim El Bakraoui were both on US Government counter-terrorism watch lists before the attacks.

AFP/Reuters