Reda Kriket, who is linked to the suspected ringleader of the Paris attacks, was arrested on Thursday in a suburb of the French capital in a raid in which police also found several assault rifles and explosives.
A specialist judge authorised the French authorities to hold Kriket beyond the usual four days without charge, a move allowed if there is an imminent risk of attack.
Kriket, a 34-year-old French national, was found guilty in absentia in Brussels in July of being part of a network recruiting jihadists to Syria and sentenced to 10 years in jail.
Also convicted in absentia in the same trial was Abdelhamid Abaaoud, the suspected ringleader of the November 13 attacks in Paris that left 130 people dead.
Investigations showed Kriket played a key role in financing the jihadist network with money from robberies and stolen goods.
Among those who went to Syria through the network were Abaaoud and another Paris attacker, Chakib Akrouh.
Separately, Dutch police on Sunday arrested a French national identified only as 32-year-old Anis B. who is thought to have been planning the attack in France in the name of the Islamic State group along with Kriket.
Dutch prosecutors said police had found ammunition in a search of the man's home in the port city of Rotterdam.
‘During the search, phones, SIM cards, hard drives, cash, ammunition and drugs were seized,’ a spokesman for the Dutch prosecutor's office, Wim de Bruin, told AFP.
No explosives were found.
The suspect, who was arrested at the request of French authorities, is expected to be extradited to France shortly.
Deepening the cross-border connections between the suspected jihadist cells, Belgian police at the weekend also charged two men with involvement in a terrorist group over the foiled French plot linked to Kriket.
One of them was named as Abderamane A., whom police shot in the leg after a stand-off at a tram-stop Friday in the Schaerbeek district of Brussels.
French Interior Minister Bernard Cazeneuve last week said the plot to stage an attack on France had been at ‘an advanced stage’, without giving further details.
European security services are under growing pressure to work together tackle the jihadist threat after 35 people were killed in last Tuesday's airport and metro attacks in Brussels.
The attack was claimed by the Islamic State group and close links have been established between the Brussels attackers and those involved in the Paris terror assaults.
