French anti-terrorism police arrested a group on Wednesday, suspecting one of them may have been planning an attack in Paris, said France's interior minister.
"We have information about one person that suggests that he could undertake violent actions in France," Interior Minister Bernard Cazeneuve said, adding the man was thought to have ties with DAESH in Syria.
"This person was arrested this morning along with people linked with him," Cazeneuve said, warning against jumping to the conclusion that an attack was imminent.
Earlier TF1 television reported that three men and one woman were arrested in Paris and the nearby northern suburb of Seine-Saint-Denis on suspicion they were planning an imminent attack.
"This is an extremely sensitive time. We've got information on people, they are preparing things, they are looking for a target, they are in France and near France," said a source close to President Francois Hollande.
During the arrests on Wednesday, police seized an unused cartridge for an automatic rifle and computer equipment, TF1 said on its website. No weapons were found, a source close to the investigation told Reuters.
The investigation was focused in particular on a 28-year-old Frenchman who had been under house arrest since Feb. 29 under a state of alert in effect in France since gunmen killed 130 people in a series of attacks last November.
The man was sentenced to five years in prison in March 2014 after he was arrested two years earlier when he tried to leave France for Syria. He was released in October 2015, TF1 reported.
On Tuesday, Belgian police shot dead a 35-year-old Algerian during a raid on a Brussels apartment in the hunt for clues to the attacks in Paris last November.