Prosecutors in Belgium have said a bomb containing nails has been found along with an Islamic State flag during police raids in Brussels this evening.

The discovery was made during a search of an apartment hours after the Belgian terror attacks, which killed at least 34 people and injured at least 200. 

"The searches that took place in the Schaerbeek (district) found an explosive device containing among other things nails," the federal prosecutor said in a statement.

"Investigators also discovered chemicals and a flag of the Islamic State," the statement added.

A series of raids are taking place across Brussels in connection with two bomb blasts at Zaventem Airport and an explosion at Maalbeek metro station.

Belgian authorities have released a photograph showing three suspects caught on CCTV cameras at the airport.

Two men seen in the photograph wearing dark jumpers are believed to have blown themselves up in the terror strike, which began around 8am local time.

A third man wearing a hat is understood to have fled the airport.

Belgian prime minister Charles Michel has said described today's events as a "dark day" for the country and declared three days of national mourning.