Belgian authorities urged the media to refrain from reporting on the probe into triple bomb attacks in Brussels on Tuesday that killed at least 26 after some said raids were under way in the city.
“The federal prosecutor notes that certain journalists are reporting information about the investigation under way,” a statement said.
“The federal prosecutor urges the press to immediately refrain from conveying information relative to the investigation so as not to harm the inquiry,” it said.
Speaking to local television, Belgian foreign minister Didier Reynders said: “We fear that people are still at large” following the attacks on Brussels airport and a busy metro station.
Federal prosecutor Frederic Van Leeuw earlier said the authorities “were doing everything possible to track down the attackers and to see if there are others who may have fled”.
Tuesday’s attacks come just four days after key Paris attacks suspect Salah Abdeslam was arrested in Brussels following a series of police raids across the city.
On Tuesday last week, police also shot dead a man linked to the November Paris attacks which left 130 people dead, with information gleaned during that raid apparently helping them track down Abdeslam.