BRUSSELS (AFP) - A third man filmed with two suicide bombers at Brussels airport who fled the scene without detonating his device, remains on the run, the federal prosecutor said Wednesday.

"The third man is on the run; he left his bag with the biggest bomb in it which exploded later because it was so unstable," Frederic Van Leeuw said, referring to the man in a hat and white coat in the CCTV images.

"This third person remains unidentified and is still being looked for," he said, adding that another man - seen in black on the left of the footage - was also unknown.

The one in the middle was Ibrahim El Bakraoui, whose brother Khalid blew himself up on the Brussels metro, he said.

The prosecutor added that El Bakraoui left a rushed will in a trash can saying he did not know what to do after police were hunting for him.

Bakraoui's will said he was "in a rush", I don't know what to do, hunted everywhere, no longer safe" and "I don't want to end up in a cell next to him", Mr van Leeuw said at a press conference.

That appeared to be a reference to Paris attacks suspect Salah Abdeslam, who is reportedly linked to Bakraoui, and who is in custody in Belgium after being captured last week.

Police investigating the Brussels attacks also found 15 kilogrammes (33 pounds) of TATP high explosives and a mass of bomb-making material during searches in the Schaerbeek district of the Belgian capital.

Mr van Leeuw said police also found 150 litres (40 US gallons) of acetone, detonators, bags filled with nails plus other equipment used to make bombs of the type used in the attacks which killed 31 people and wounded another 270.

TATP high explosives have been used extensively by Islamic State terrorists.

This story is developing.