A judge has ordered Mohamed Abrini and five other suspects arrested in connection with the attacks in Paris and Brussels to be held for another month, Belgian prosecutors said.
Abrini has admitted being the "man in the hat" seen leaving Brussels Airport the morning two suicide bombers detonated explosives-laden suitcases, killing 16 people.
Another 16 died that morning when a bomber blew himself up on a Brussels subway train.
France's BFM television reported Abrini had claimed to investigators he "wouldn't hurt a fly".
Abrini also had close ties to the attackers who killed 130 victims on November 13 in Paris, and is believed to have travelled to Syria and to have met with suspected extremists in the UK.
Belgian and French authorities have detained dozens of suspect in their investigations.
Earlier, Belgium's transport minister came under pressure to resign over a secret European Union report exposing past lapses in oversight of security at airports across the country.
The restricted EU document from last year was made public by two opposition parties.
It said that oversight of security measures was flawed and there were serious deficiencies in the way safety checks were managed.
Transport Minister Jacqueline Galant did not immediately comment.
Opposition parties are set to question the minister during a parliamentary session.