Friends said he was in the Brussels area and trying to get to Syria.
The city was a base for the attackers - Islamic State militants - who killed 130 people in Paris.
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Prime Minister Charles Michel said later there had been "quite precise information" that "several individuals with arms and explosives could launch an attack... perhaps even in several places".
Saturday's developments as they unfolded
Brussels terror threat: 'Everyone is on edge'
Belgium's jihadist networks
Paris attacks caused archbishop 'doubt'
One of the men who drove Salam Abdeslam to Belgium told his lawyer that he was dressed in a "big jacket" and may have had a suicide belt.
The lawyer, Carine Couquelet, told Belgian TV this raised questions, including the possibility that Salah Abdeslam may have been supposed to blow himself up in Paris but had had second thoughts.
Friends of Abdeslam told ABC News they had spoken to him on Skype and said he was hiding in Brussels and desperately trying to get to Syria.
They said he was caught between European authorities hunting him and so-called Islamic State members who were "watching him" and were unhappy that he had not detonated his suicide belt.