Belgium has revealed that the Isis militants who bombed the Zaventem airport and the metro train on Monday, killing more than 30 people, were brothers.

Ibrahim El Bakraoui, 29, and Khalid El Bakraoui, 27, were both born in Belgium and had a criminal record but the police had not linked them to Islamist militants until the arrest of a key suspect in the Paris bombings.

Meanwhile, Turkey said it had warned Belgium about Ibrahim El Bakraoui's militant links but Brussels ignored the warning.

Turkish President Tayyip Erdogan said Bakraoui was detained and deported to the Netherlands.

"One of the attackers in Brussels is an individual we detained in Gaziantep in June 2015 and deported. We reported the deportation to the Belgian Embassy in Ankara on July 14, 2015, but he was later set free," Erdogan said, according to Reuters.

"Belgium ignored our warning that this person is a foreign fighter," Erdogan said.

He said Turkey had also informed Dutch authorities about Bakraoui's militant links. A Dutch government official said they were investigating if Bakraoui was in the Netherlands.

Erdogan's office said Belgian authorities released Bakraoui as they could not find any evidence linking him to terrorism.

Belgian prosecutors said the Bakraoui brothers were identified as the bombers by their fingerprints and on security footage.

The officials are yet to identity the second suicide bomber. They are on the lookout for a third terrorist named Najim Laachraoui, who fled the scene without detonating himself.

Belgium's federal prosecutor said police recovered a computer containing Ibrahim El Bakraoui's will from a rubbish bin his area.

Bomber's will

In the will, the militant described himself as "always on the run, not knowing what to do anymore, being hunted everywhere, not being safe any longer and that if he hangs around, he risks ending up next to the person in a cell".

Police found a nail bomb and an Islamic State flag from an apartment in Schaerbeek, from where the suspected bombers came.

Explosives and Islamic flags were found during a raid in southern Brussels last week to flush out terrorists involved in the Paris attacks in November that killed 130 people.