A leading White House coronavirus expert warned on Sunday that the rising number of cases in the US was making it harder to determine how people were contracting the virus, intensifying concern over people attending large events around the country.

Anthony Fauci, the widely respected head of the US National Institute of Allergy and Infectious Diseases, said health authorities were “getting a better sense” of the scope of the coronavirus outbreak across the country. But he added that he was worried about the growing incidence of “ community spread” — where the source of the virus is unknown.

“We’re getting a better sense [of the scope] as the days go by. Unfortunately, that better sense is not encouraging because we’re seeing community spread,” Mr Fauci told NBC News. “Whenever you see ‘community spread’, you can do contact tracing, but as more communities spread it, it becomes logistically more difficult.”