Centers for Disease Control and Prevention (CDC) director Robert Redfield said the danger was higher as a fresh outbreak would likely coincide with the flu season.

It would put "unimaginable strain" on the US health care system, he said.

The US has seen more than 800,000 cases - the highest in the world.

More than 45,000 people have so far died with coronavirus across the US, according to a tally kept by Johns Hopkins University.

California had its highest one-day rise in new cases on Monday while New Jersey, the worst-hit US state apart from New York, saw its highest increase in deaths in one day.

In other developments in the US:

• President Trump says he will halt applications for US green cards - which give immigrants legal permanent residence and the opportunity to apply for American citizenship - for 60 days
• Harvard University says it will keep an $8.6m-coronavirus grant despite pressure from President Trump to return it
• California's first cases of coronavirus occurred much earlier than previously thought, health officials have confirmed. A post-mortem examination has revealed that a person who died at home on 6 February in Santa Clara county is now the first known fatality in the US. Until now, the first fatality was thought to have been a man in Washington state who died on 26 February
• The US Senate has unanimously approved $484bn (£392bn) in coronavirus relief, including funds designed to help small businesses. The legislation will go to the House of Representatives for approval on Thursday

In an interview with the Washington Post, Mr Redfield said that "there's a possibility that the assault of the virus on our nation next winter will actually be even more difficult than the one we just went through".

He urged officials in the US to prepare for the possibility of having to confront a flu and a coronavirus epidemic at the same time.

'Really, really difficult'

Mr Redfield stressed the importance of getting flu shots. He said that getting vaccinated "may allow there to be a hospital bed available for your mother or grandmother that may get coronavirus".

The CDC chief said that coronavirus had arrived in the US as the regular flu season was subsiding. He argued that if it had arrived at the peak of the winter influenza season, "it could have been been really, really, really, really difficult".

His warning comes as several US states are moving to ease lockdown restrictions.

Mr Redfield said that social distancing remained key to curb the spread of the virus and urged officials to keep stressing its importance even as restrictions on movement were lifted.

He said recent anti-lockdown protests were "not helpful".

Image copyright Alyson McClaran

 At an anti-lockdown protest in Denver a man wearing scrubs challenged demonstrators

He urged state and federal officials to step up tests to identify those who have coronavirus and the people they have had contact.

He said that the Centers for Disease Control planned to hire more than 650 people - more than doubling its current staff - to help with contact tracing among other things.

The CDC was also exploring the possibility of using Census Bureau workers to help with contact tracing he said.