The woman sprayed the tourists in the eyes, the source said, adding that she targeted the people closest to her and that the incident "was not of a terrorist nature".
The spokeswoman spoke on condition of anonymity, per the custom of the French judicial system.
People once used knives, now acid is a weapon of choice in the United Kingdom .
United States authorities in France are not immediately commenting further on what happened to protect the privacy of the American tourists, embassy spokesman Alex Daniels said.
The attacker, a 41-year-old woman, was arrested at the scene, Reuters reported.
French reports say the incident is not being treated as terror-related and witnesses said no slogans were shouted during the attack.
The incident comes just days after a homemade bomb planted in a rush-hour subway auto injured 29 people in London, sparking a huge manhunt for the perpetrators of what police said was the fourth terrorist attack in the British capital this year.