Brussels attacker already on US watch list

Two brothers involved in the Brussels suicide attacks had been on a US terror watch list, official sources say.

Two brothers involved in the Brussels suicide attacks had been on a US terror watch list. (AAP)

Brahim El Bakraoui, one of the Brussels suicide bombers, was on a US counter-terrorism watch list before the November attacks in Paris and his brother Khalid was put on the list shortly afterward, sources familiar with the matter say.

Reuters previously reported that both brothers had been known to US authorities before the March 18 arrest of Salah Abdeslam, a French national who prosecutors say had a key role in the Paris attacks.

Belgian prosecutors have identified Brahim El Bakraoui as one of two suicide bombers who attacked Brussels' Zaventem Airport on Tuesday, while they say Khalid El Bakraoui carried out a suicide bombing at Brussels' Maelbeek Metro station, near European Union headquarters.