NEW YORK - Once the epicentre of the coronavirus outbreak in the US, New York has successfully brought the number of cases down.

But as restrictions begin to relax, young New Yorkers are flouting social distancing rules and partying as if the pandemic never happened, threatening to create a second wave of infections.

New York City entered a limited fourth phase of reopening last week, allowing some art and entertainment venues such as zoos and botanical gardens to open for outdoor activities at a limited capacity.

But it has paused the reopening of venues for indoor activities such as gyms, malls, movie theatres, museums and indoor dining places.

As the lockdown eases, people have been seen flouting social distancing rules and gathering outside restaurants and bars sans masks.

Such behaviour threatens to undo the work in stemming the spread of the coronavirus, which at its height saw more than 14,000 people in hospitals and a daily death toll of more than 1,000.

On Thursday (July 23), the state reported 811 new cases, with a positive rate of 1.16 per cent, and 13 deaths.

Though new cases, hospitalisations and deaths have declined, positive coronavirus tests are on the rise for people in their 20s and 30s.

New York Governor Andrew Cuomo said at a briefing on Thursday that the proportion of people aged 21 to 30 who tested positive for the virus had increased from 9.9 per cent to 13.2 per cent over the past two weeks.

One cluster in Albany, the capital of New York state, saw nearly 30 positive cases linked to a Fourth of July party.

To crack down on outdoor mingling, new rules were announced banning establishments from selling alcohol to customers who do not also buy food.

On Wednesday, the state government suspended 27 licences and brought 410 charges against bars and restaurants which allowed large crowds to gather and violated social distancing regulations, according to ABC7.

Mr Cuomo on Monday warned of a potential rollback on the reopenings of bars and restaurants if the gatherings of young people across the state were "getting worse".

"Don't be stupid. What they are doing is stupid and reckless for themselves and for other people. And it has to stop," he said.

The outbreak in the United States remains on a troubling trajectory, surpassing the grim milestone of four million infections on Thursday (July 23), when more than 1,000 deaths from Covid-19 were also recorded for the third straight day.

Fatalities nationwide stood at 1,014 on Thursday, with not all states reporting. There were 1,135 deaths on Wednesday, and 1,141 on Tuesday.

The country took 98 days to reach one million confirmed cases of Covid-19, but just 16 days for the figure to increase from three million to four million, according to a Reuters tally.

The average number of new cases is now rising by more than 2,600 per hour nationwide, the highest rate in the world.

President Donald Trump told a White House briefing on Thursday that hot spot states may need to delay the reopening of schools by a few weeks, but pushed for most students to be able to return to classrooms in the autumn.

"They have to open," he said, as quoted by Reuters.

The US Centres for Disease Control and Prevention issued additional guidelines on how schools can safely reopen. These include a call for students to stay 2m apart from one another where feasible, and to bring meals from home

and eat in classrooms instead of the lunchroom.

Schools were closed across the country after the coronavirus emerged and began spreading, and Mr Trump has been determined to find a way to get them to open again.

Despite pressure from Mr Trump, only one in four Americans thinks it is safe for public schools to reopen this autumn, and four in 10 parents said they would likely keep their children home if classes resume, according to a Reuters/Ipsos poll last week.