President Donald Trump has put Vice-President Mike Pence in charge of coordinating the United States' response to the coronavirus outbreak, as the health authorities said they have identified the first Covid-19 case that does not have ties to a known outbreak.
It is a worrying signal that the virus is already circulating despite reassurances from the Trump administration it has been contained.
But the President also sent mixed messages that were not helpful, including on the inevitability of the virus' spread within America and speed of developing a vaccine, said public health experts.
"Because of all we have done, the risk remains very low," Mr Trump said at a rare news conference in the White House briefing room on Wednesday.
"We are very, very ready for this, whether it is going to be a breakout of larger proportions or whether or not we are at that very low level."
Of the more than 82,000 cases worldwide, 15 were detected in the US. There are 45 more cases of Americans with the virus: Three who were repatriated from Wuhan, and 42 passengers from the Diamond Princess cruise ship.
Mr Trump and top health officials defended his administration's early response of quarantining exposed individuals and temporarily banning foreigners who had recently been to China from entering the country, crediting it with keeping the number of cases in the US low.
He did not rule out further restrictions on travellers from other countries hit by the virus, including South Korea and Italy, when asked.
Flanking Mr Trump at the news conference, US Centres for Disease Control and Prevention (CDC) principal deputy director Anne Schuchat said: "Our aggressive containment strategy has been working and is responsible for the low level of cases we have so far. However, we do expect more cases, and this is a good time to prepare."
Immediately after, Mr Trump implied he disagreed, and took issue with remarks from the CDC a day earlier that the spread of the virus in the US was a matter of when, not if.
"I don't believe it is inevitable. It is possible. It could be at a smaller level or a larger level," said Mr Trump. "There is a chance it would get fairly substantially worse. But nothing is inevitable."
Infectious diseases expert and former CDC head Tom Frieden said the mixed messages were unhelpful. "Yes, the US is doing well now and the risk is low now," he said on Twitter. "But a pandemic is inevitable, and thinking we can keep it out of the US is a dangerous delusion."
Shortly after Mr Trump's conference, the CDC announced that a patient in California had tested positive for the virus despite having no history of travel to countries with the virus and no known contact with other patients. The unknown origins of the case mean it could be the first known instance of community spread in the US, said experts.
Georgetown University public health law professor Lawrence Gostin said: "Trump implies no likely community spread in the US. Yet, the CDC just informed of the first Covid-19 case in a person who didn't return from a foreign country or have contact with a confirmed case."
Mr Trump also said a vaccine could be developed "fairly rapidly", and was "coming along very well".
But shortly after, National Institute of Allergy and Infectious Diseases head Anthony Fauci said a vaccine was one to 1½ years away, although this would still be the US' fastest turnaround from sequencing a virus to a large-scale vaccine trial.
The White House requested US$2.5 billion (S$3.5 billion) from Congress on Monday to fight the coronavirus. House Democrats have deemed the sum insufficient, and said it should be closer to US$8.5 billion.