A third of the nearly 2,000 sailors who were aboard a French aircraft carrier and support craft when a coronavirus outbreak occurred at sea have tested positive.

With two-thirds of test results in, 668 sailors from the Charles de Gaulle and escort vessels in its battle group are confirmed to have COVID-19, France's defence ministry announced on Wednesday.

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Thirty-one were being treated in hospital, and one was in intensive care, a ministry statement said.

Last week, the Charles de Gaulle was brought back 10 days early from a deployment in the Atlantic after some crew members showed coronavirus symptoms.

The carrier, which had helicopters and fighter jets on board, was accompanied by two frigates - one for aerial defence and the other an anti-submarine vessel.

Sailors from the Charles de Gaulle and one of the frigates - and the pilots who returned the aircraft to their respective bases - are all placed in isolation for 14 days, the ministry said.

There had been no virus outbreak on the other frigate.

So far, 1,767 sailors from the battle group have been tested for the virus, the vast majority from the aircraft carrier itself, said the ministry statement.

The Charles de Gaulle, which can transport about 2,000 sailors, had been deployed in the Atlantic as part of a NATO exercise after taking part in Operation Chammal, which seeks to contain the ISIS (ISIL) group in Iraq and Syria.

The vessels in the battle group are being disinfected, the ministry said.