US cases of the novel coronavirus have topped 1 million, having doubled in 18 days, according to a Reuters tally.

More than 56,500 Americans have died of Covid-19, an average of about 2,000 a day this month, according to the tally.

About 30 per cent of the cases have occurred in New York state, the epicentre of the US outbreak, followed by New Jersey, Massachusetts, California and Pennsylvania.