French authorities were working on Friday to contain a fuel spill from the sunken Grimaldi ro-ro Grande America.
The 57,000-gt ship (built 1997) went down 330km west of La Rochelle earlier this week following a fire on deck.
French and British rescue teams saved all 27 crew.
Images released by the French navy this week showed flames and plumes of black smoke spewing from the ship as it listed sharply before sinking.
AP cited the regional maritime authority as saying the ro-ro has since leaked fuel over an area of about 10km by 1km.
The vessel was carrying about 2,200 tons of fuel before the accident, the French government minister in charge of the environment, Francois de Rugy, told BFM television.
A French clean-up ship had been due to arrive on Thursday.
France has asked the European Maritime Security Agency for help using satellites to locate leaks.
The regional prosecutor has opened an investigation.
Claims could top $50m
Norwegian Hull Club (NHC) is the lead hull and machinery underwriter covering the vessel.
The incident is expected to generate total claims in excess of $50m.
Compatriot Gard is also among the list of hull insurers that will be paying out on the loss which was initiated by a fire in a deck container.
The ship was heading to Morocco from Germany.
Experts suggested it is likely the sinking followed fire-extinguishing water causing free surface effect on the ship’s deck and putting the vessel into a list from which it could not recover.
