Brussels airport suicide bombers were brothers El Bakraoui known to police,  says Brussels public broadcaster RTBF.

Belgian police are hunting an Islamic State suspect seen with two supposed suicide bombers shortly before they struck Brussels Airport in the first of two attacks that also hit the city's metro, killing at least 30 and wounding over 200.

The blasts on Tuesday claimed by the Syrian-based militants four days after the arrest in Brussels of a prime suspect in November's Paris attacks, sent shockwaves across Europe and around the world, with authorities racing to review security at airports and transit systems, and drawing an outpouring of solidarity.

Investigators said they were focusing on a man in a hat who was caught on CCTV pushing a laden baggage trolley at the airport with two others they believed were the bombers. An unused explosive device was later found at the airport and a man was seen running away from the terminal after the explosions.

Security experts believed the blasts, which killed about 20 on a metro train running through the area that houses European Union institutions, were probably in preparation before Friday's arrest of locally based French national Salah Abdeslam, 26, whom prosecutors accuse of a key role in the Nov. 13 Paris attacks.

"A photograph of three male suspects was taken at Zaventem. Two of them seem to have committed suicide attacks. The third, wearing a light-colored jacket and a hat, is actively being sought," prosecutor Frederic Van Leeuw told a news conference.

The two men in dark clothes wore gloves on their left hands only. One security expert speculated they might have concealed detonators. The man in the hat was not wearing any gloves.