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.
At the White House, Mr Trump said 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 for everyone should be from 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."
White House coronavirus response co-ordinator Dr Deborah Birx, who joined the president, said: "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.