Miami will fine people up to £400 for not wearing a face mask in public.
Following a surge in coronavirus cases in Florida, Miami issued the emergency order at the end of last week.
Everybody will be required to wear a mask in public spaces in the city.
However, people with disabilities that prevent them wearing a mask, people exercising outdoors with appropriate social distancing and children under the age of two will be exempt.
Those caught breaking the order will be handed a free mask and given a verbal warning.
But if they are caught again flouting the order they will be fined £40. A third offence will cost people £120 and a fourth and final time will land them with a £400 fine and a visit to court.
Florida Governor Ron DeSantis attributed the soaring number of people infected to widespread noncompliance with the state’s reopening guidelines.
Beaches in Miami, Fort Lauderdale and Palm Beach, popular with residents and tourists are to be closed for the upcoming Fourth of July holiday.
County Mayor Carlos Gimenez said that the closure will run from July 3-7, but could be extended “if conditions do not improve and people do not follow New Normal rules requiring masks to be worn always inside commercial establishments and outdoors when social distancing of at least 6 feet is not possible”.
He added: “As we continue to see more COVID-19 positive test results among young adults and rising hospitalizations, I have decided that the only prudent thing to do to tamp down this recent uptick is to crack down on recreational activities that put our overall community at higher risk.”
At least 12 states have by now suspended or rolled back plans to reopen, hoping the move will slow the further spread of the virus.
Only on Sunday, US Health and Human Services Secretary Alex Azar warned the “window is closing” for the country to get the pandemic under control.
There are at least 2,549,069 cases of coronavirus in America, and at least 125,803 people have died, according to Johns Hopkins University’s tally.
Last Friday, the US recorded its biggest single-day increase in cases with 40,173.
In the past week, officials and state leaders have made repeated pleas to Americans to stay away from crowded spaces, keep their distance and try to stay at home as much as possible.
In California Governor Gavin Newsom was forced to order seven counties in and around Los Angeles, to close bars as cases surged.
Newsom, the first governor to institute a statewide-stay-at-home order in March, also recommended nine other California counties, including Sacramento, do the same.
The surges come as a huge blow to Donald Trump, who has pushed for States to reopen amid criticism of his handling of the disease.
At least 31 states are now seeing an increase in new cases compared to the previous week, according to John Hopkins University.
Only four – Connecticut, Delaware, New Hampshire and Rhode Island – are seeing a decline.