President Donald Trump has warned Americans to brace for a "rough two-week period" ahead as the White House released new projections that there could be 100,000 to 240,000 deaths in the US from the coronavirus pandemic even if current social distancing guidelines are maintained.

Public health officials stressed that the number could be less if people change their behaviour.

"We really believe we can do a lot better than that," said Dr. Deborah Birx, the coordinator of the White House coronavirus task force. That would require all Americans to take seriously their role in preventing the spread of disease, she said.

Trump called American efforts to slow the spread of the coronavirus "a matter of life and death" and urged the public to heed his administration's guidelines. He predicted the country would soon see a "light at the end of the tunnel" in the pandemic that has killed more than 3,500 Americans and infected 170,000 more.

"I want every American to be prepared for the hard days that lie ahead," Trump said. "We're going to go through a very tough two weeks."

Dr. Anthony Fauci, the government's top infectious disease expert, said the numbers are "sobering" and called on Americans to "step on the accelerator" with their collective mitigation efforts.

"We are continue to see things go up," Fauci said. "We cannot be discouraged by that because the mitigation is actually working and will work."

Birx said pandemic forecasts initially predicted 1.5 million to 2.2 million deaths in the US But that was a worst-case scenario, without efforts to slow the spread of the coronavirus through social distancing.

Birx said states that have not yet seen a spike in cases as New York has could take action to flatten the curve of rising hospitalisations and deaths.

As for the projection of 100,000-240,000 deaths, Fauci of the National Institutes of Health said: "We don't accept that number, that that's what it's going to be. ... We want to do much better than that."

The comments came after Trump announced Sunday that he was extending to April 30 the social distancing guidelines that urged Americans to cease social gatherings, work from home, suspend onsite learning at schools and more in a nationwide effort to stem the spread of the virus.

It was an abrupt reversal for Trump, who spent much of last week targeting April 12 as the day he wanted to see Americans "pack the pews" for Easter Sunday services.

Many states and local governments already have stiffer controls in place on mobility and gatherings.

Trump spoke after another troubling day for the stock market, which has been in a free fall as the cononavirus ground the economy to a near-halt and left millions unemployed. The Dow Jones Industrial Average plunged more than 400 points, or roughly 1.9 per cent, to seal the worst first-quarter finish of its 135-year history.