US President Donald Trump has said the nationwide coronavirus emergency could last until the end of the summer or even longer.
He said Americans over the next 15 days should not gather in groups of more than 10 and avoid bars, restaurants, food courts, gyms and crowds.
Mr Trump said at the White House the country is facing “an invisible enemy” that is “so contagious”.
The US has so far had more than 4,200 cases of the virus and 74 deaths.
There have been more than 174,000 confirmed cases of coronavirus globally and over 6,700 deaths, according to a tally from Johns Hopkins University.
Under the US coronavirus task force’s other new guidelines announced by Mr Trump on Monday:
All older Americans are urged to stay home, while work and schooling should be at home
Discretionary travel, shopping trips and social visits should be avoided and people should stay away from nursing homes or retirement facilities
Anyone in a household who tests positive for the virus should remain at home along with everyone who lives there
“We’ve made the decision to further toughen the guidelines and blunt the infection now,” Mr Trump told reporters.
“We’d much rather be ahead of the curve than behind it.”
Asked how long the emergency would last, Mr Trump told reporters: “They think August, could be July, could be longer than that.”
He said he was not considering a national curfew or lockdown, though added: “We may look at certain areas, certain hot spots as they call them.”
Mr Trump told reporters he had not yet decided to close the US-Canada border, but said it was something the administration was considering.
As stocks tumbled, Mr Trump acknowledged a recession was possible, but said: “The market will take care of itself.”
As the White House works on a financial assistance package for the beleaguered US airline industry, Mr Trump said it was right to help them.
“It’s not their fault,” he said. A trade group for major airlines said the industry requires more than $50bn (£40bn) in aid to survive.
The president also addressed issues of testing, as the US has been criticised for lagging far behind smaller countries in getting tests out to the states.
Officials on Monday said a million tests were currently available and more would be coming this week.
“A lot of testing has been going on,” Mr Trump said, though he also noted that those without symptoms should not seek a test.
“Not everybody should run out and get the test, but we’re able to handle tremendous numbers.”
The president, who has been criticised for initially playing down the seriousness of the virus, was asked by a reporter how he would score his administration’s response to the crisis on a scale of one to 10.
Mr Trump said: “I’d rate it a 10. I think we’ve done a great job.”
White House coronavirus response co-ordinator Dr Deborah Birx, who joined the president, told reporters about the new guidelines: “If everybody in America does what we ask for over the next 15 days, we will see a dramatic difference.”
She issued an appeal directly to millennials, asking them to limit social contact even though they are at lower risk of suffering if they contract the virus.
“They are the core people that will stop this virus,” she said. “We really want people to be separated.”
Dr Birx also warned against socialising even if people feel well.
“We know that there is a large group of infected people who are asymptomatic, who continue to spread the virus,” she said.
Vice-President Mike Pence, who is leading the coronavirus taskforce, told reporters he had not been tested yet.
How are US states responding?
At least 40 US states have declared states of emergency over the virus.
New York, New Jersey, Pennsylvania, the District of Columbia, Ohio, California, Illinois, Michigan, Massachusetts and Washington state have shut or enacted restrictions on bars and restaurants.
Twenty-nine states have announced that they are closing schools.