We don't know more about intentions of attacker, who has been arrested. The individual was not known to the police, the source added.
Police spokeswoman Johana Primevert told The Associated Press that the incident took place early Friday in the Chatelet station in central Paris.
The suspect, said to be shouting Islamic State slogans, attacked a French soldier carrying out anti-terrorism duties in the French captial this morning.
"The man is under control".
Prosecutors said they were treating the incident as a possible terror attack, but also did not rule out the possibility the attacker was deranged.
Further noting that the soldier who was targeted by the assailant, was a member of Operation Sentinel, a task force that has been assigned to protect prominent sites in the region.
French police have one terrorism suspect in custody and are on a manhunt for another terror suspect after two separate terror attacks today.
On August 9, at least six French soldiers were injured, three of them seriously, when they were hit by a vehicle in a north-western Paris suburb.
France has been under a state of emergency since a series of coordinated attacks in Paris in November 2015 which left 130 people dead.
French armed forces minister Florence Parly said: "A man has been arrested".
In April, a police officer was killed in a shooting on the Champs-Elysées.