A US military hospital ship left New York on Thursday after a month in the city as coronavirus cases in America's epicentre continue to decline, AFP reports.

The USNS Comfort sailed out of a Manhattan pier shortly after midday in a low-key departure that contrasted sharply with the fanfare of its arrival.

President Donald Trump dispatched the 1,000-bed ship to the Big Apple as projections suggested New York state might need to double hospital beds to 110,000 for the peak of its coronavirus outbreak.

The ship was part of a massive operation to boost hospital capacity that included converting the Javits Convention Center and the home of the US Open tennis tournament into emergency field hospitals.

Tents for patients were also erected in Central Park.