A French judge has put under formal investigation a Frenchman arrested last week on suspicion of planning an imminent act of "extreme violence", Paris' prosecutor said.

Francois Molins said the "unprecedented" amount of weapons, including five assault rifles and handguns as well as chemicals and explosives that could be used for a bomb, had been found at Reda Kriket's apartment.

Police also found in his apartment five false passports, brand-new mobile phones and two computers which contained information about bomb-making and jihadist groups.

"Everything suggests that the discovery of this cache avoided an act of extreme violence by a terrorist network," Molins told a news conference.

The 32-year-old, who was sentenced in absentia to 10 years in prison by a Belgian court last July for recruiting Islamist fighters for Syria, said in questioning by French investigators that he was not a terrorist but also gave up little information, Molins said.

Kriket is suspected of having spent time in Syria in late 2014 and early 2015 along with another man, Anis Bahri, who was arrested in Rotterdam on March 27 at the request of French police and is expected to be extradited to France.

French investigators have been tracking Kriket and Bahri since the attacks in and near Paris in November in which suicide bombers and gunmen killed 130 people.

Kriket's arrest on March 23 came two days after suicide bombers hit the Brussels airport and a metro train, killing 32 people in the worst such attack in Belgian history.

Molins said French authorities were also working with their Belgian counterparts on investigations of two people linked to Kriket, whom he named as Abderrahmane A. and Rabat M.

Abderrahmane was convicted in 2005 by a Paris court for having played a role in the assassination of Afghan resistance commander Ahmad Shah Massoud, who was killed by al Qaeda a few days before the 2001 terrorist attacks in the United States.