WASHINGTON - President Donald Trump named Vice-President Mike Pence in charge of coordinating the United States' response to the coronavirus outbreak on Wednesday (Feb 26).

He also reassured the American public that the risk of the virus remained low even as the epidemic spread across Europe and the Middle East.

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," said Mr Trump at a rare news conference in the White House briefing room.

"We are very, very ready for this, whether it's 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 who were passengers aboard the coronavirus-stricken 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 total 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 press 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's inevitable. It's possible. It could be at a smaller level or a larger level," said Mr Trump.

"There's 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.

"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," said Georgetown University public health law professor Lawrence Gostin, using the coronavirus disease's official name.

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 still one to 1½ years away, although this would still be America's 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, including developing a vaccine and stockpiling personal protective equipment.

House Democrats have deemed the sum insufficient and said that it should be closer to US$8.5 billion.

"We'll be satisfied whatever it is," said Mr Trump on Wednesday.