The French Defence Ministry has reported that at least 40 sailors aboard an aircraft carrier have shown symptoms of the new coronavirus.
The vessel, Charles de Gaulle, headed back to port on Wednesday due to the possible COVID-19 outbreak.
In a statement, the `Ministry of Armed Forces added that "some forty sailors are now under reinforced medical observation".
"The French Navy and the command of the aircraft carrier are closely monitoring the evolution of the health status of the sailors," it added.
French government spokeswoman Sibeth Ndiaye added the suspected patients "have been placed in isolated confinement out of precaution".
A medical team equipped with test kits was set to board the Charles de Gaulle to confirm whether any crew members were infected, and prevent any potential spread of the virus.
Gatherings on the ship have been limited and health procedures, including the disinfecting of shared spaces and equipment, have been reinforced.
The Ministry of Armed Forces also stated that masks have been distributed "as a preventive measure" to all staff who may present symptoms.
There are around 20 medical staff on board the carrier, including doctors, nurses and surgeons, as well as a dedicated hospital ward.
The vessel also has helicopters, which can be used to evacuate patients if necessary, according to the ministry.
More than 1,700 crew members are currently on board the Charles de Gaulle, the flagship of the French navy.
The aircraft carrier was on a mission in the Atlantic Ocean, and is returning immediately to its base in the port of Toulon, on the Mediterranean coast.
It was originally expected to dock there on April 23, but no new arrival date has been confirmed.
A coronavirus outbreak on the USS Theodore Roosevelt led to the dismissal of the ship's captain last week, and the resignation of the US Navy Secretary.