Security agencies in Paris have circulated a photo of an Afghan asylum seeker on suspicion that he might be plotting an attack on the capital, a police source said.
It comes as France reels from two strikes in a month by Islamic State loyalists.
Police did not have a name for the Afghan, the source said, and no active manhunt was underway. Metronews reported that the man had been in France for the past two months.
France has been under emergency rule since militant gunmen and suicide bombers killed 130 people in Paris in November.
Last month, a Tunisian drove his truck into Bastille Day revellers in Nice, killing 84 people, and two Islamist militants cut the throat of an elderly priest.
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Days before the Normandy church attack on July 26, France's security services had received a tip-off from a foreign intelligence agency that a suspected militant was planning an attack, and sent out a nameless photo to different agencies.
The photo turned out to be of one of the church killers, 19-year-old Abdel-Malik Nabir Petitjean.
Paris is on high alert, wary in particular of the potential for an attack on the "Paris Plage" beach festival.
Every summer Paris closes off a major road along one bank of the river Seine and dumps truckloads of sand to create an artificial beach.
Following the Nice attack, vehicles and concrete blocks are being used to block entrances to the site.
More than 50 festivals and events have been scrapped countrywide following last month's attacks, including fireworks displays to mark the Catholic feast of the Assumption of the Virgin Mary on August 15.
