The French navy is investigating how the novel coronavirus infected more than 1,000 sailors aboard the aircraft carrier Charles de Gaulle.

Key points: The infections have immobilised the ship, which is being thoroughly cleaned with hot water and an anti-viral product

The infections have immobilised the ship, which is being thoroughly cleaned with hot water and an anti-viral product Despite this, French Defence Minister Florence Parly insists the country is ready to defend itself

Despite this, French Defence Minister Florence Parly insists the country is ready to defend itself A Navy spokesman said it was difficult to apply social distancing measures on a combat vessel

The ship, France's biggest carrier and the flagship of its navy, has been undergoing a lengthy disinfection process since returning to its home base in Toulon five days ago.

One person remains in intensive care and some 20 others hospitalised, navy spokesman Commander Eric Lavault said.

Two of four US sailors serving aboard the Charles de Gaulle as part of an exchange program also tested positive, according to a US Navy statement. A British sailor was aboard another vessel, Commander Lavault said, refusing to reveal the sailor's health status.

Commander Lavault insisted that the aircraft carrier's commander sought to increase the physical distance among the crew on the vessel, where there was no testing equipment and, for most of its three months on operations, no masks.

It is "very difficult to apply social distancing measures … on a combat vessel," he said. But "security of the crew is the first concern. A combat ship, especially an aircraft carrier, is nothing without its crew".

There have been 1,081 infections out of 2,300 people on the Charles de Gaulle and its escort vessels. ( AP: Y Bisson/ Marine Nationale )

A similar outbreak on the USS Theodore Roosevelt and a dispute about how the at-sea health crisis was handled led to the firing of its captain and the resignation this month of the acting US Navy secretary.

The French navy has been spared major controversy so far, but the Defence Minister and the head of the French military's health service arm were questioned about the infections at parliamentary hearings.

Defence Minister Florence Parly told MPs that 1,081 of the 2,300 people aboard the Charles de Gaulle and its escort vessels have tested positive so far — nearly half the overall personnel.

While the virus has immobilised the immense and important ship, Ms Parly insisted that otherwise "our forces continue to assure the defence of our country at sea, under the sea, on land and in the air".

An investigation to retrace the paths of the personnel is in progress.

Commander Lavault noted that the aircraft carrier made a call in the French port of Brest, on the Atlantic Ocean, had been in the North Sea as part of a "naval diplomacy" mission with NATO partners, and had stopped in Cyprus during an operation in the eastern Mediterranean Sea to join in the fight against the Islamic State group.

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Journalists had boarded the vessel at one point.

"All hypotheses are on the table," Commander Lavault said.

The Defence Minister defended the decision to allow the ship to stop in Brest in mid-March, even though France had already ordered all schools closed to fight the virus and the government was preparing confinement measures.

Hours after the ship left, President Emmanuel Macron announced a nationwide lockdown, among the strictest in Europe.

The aircraft carrier was back in its home base at the Mediterranean port of Toulon on April 12.

Commander Lavault said the carrier was being cleaned top to bottom, first with high-pressure water at 60C, then with an anti-viral product, a process that could take weeks. He said the goal is to get the carrier back to sea sometime in May.

France has been among the countries hardest hit by the virus, with more than 18,000 deaths confirmed as of Friday (local time) and more than 100,000 reported cases.

As France heads into a second month of confinement, the head of the national health agency said that the country was seeing "a slow but regular decline" in the virus, with the number of hospitalisations and people in intensive care continuing to drop.

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AP