The suspect was identified as Abdelhamid Abaaoud, 27, who was born in Belgium to Moroccan parents, and raised in the working-class Brussels district of Molenbeek.
Several reports claimed that he remains in Greece or fled to neighboring Turkey.
Abaaoud is believed to have travelled to Syria and fought in the ranks of ISIS. Moreover, he has been identified as the driver behind the wheel of a SUV dragging a corpse on a Syrian battlefield, images that were first aired in March 2014.
According to the French-language newspaper La Derniere Heure, Belgian authorities are hunting for the alleged jihadi cell leader and financier with the help of the FBI.
The group was preparing to attack police stations in the country before a police raid on Thursday evening, press reports state.
The Flemish television network VTM said surveillance of terror cell members began between Christmas and New Year’s Eve, following several suspicious phone calls received by an inmate incarcerated at the Lantin prison, in Liege.
Belgian authorities said the inmate in question served as Abbouad’s “liaison” with suspected jihadis in Belgium. The inmate was also identified as the brother of one of the two suspects killed Thursday evening during the police raid in the eastern Belgian town of Verviers.
VTM said the telephone calls were traced to Greece, whereas La Dernier Heure adds that Abaaoud (whose nom de guerre is Abu Omar Soussi) may be in Turkey.
The television network adds that Belgian authorities have contacted their Greek counterparts over the investigation.
Belgium’s federal prosecutor’s office neither confirmed nor dismissed the information, in response to a question by AFP.