A royal decree announcing the freeze says the 15, including alleged organizer Oussama Atar, are "suspected of committing or attempting to commit terrorist acts; to aid or participate in them."
The decision, which took effect yesterday, also imposes a ban on directly or indirectly providing funds or economic resources to them.
"We are going to systematically extend this list in the months and years to come," Justice Minister Koen Geens told lawmakers today.
Justice and security officials confirmed yesterday that Atar's name appears in the investigation into the attacks, which killed a total of 162 people. Atar is suspected to be the real identity of Abu Ahmed, a pseudonym of the suspected attack coordinator. His whereabouts are unknown.
Atar's name has been circulating in Belgium for months as possibly linked to the attacks but Belgian prosecutors have declined to comment on it.