President Donald Trump on Wednesday said it was not inevitable that the novel coronavirus would continue to spread in the United States, as he said he is considering travel restrictions on Italy and South Korea and appointed his vice president to lead the response.

His optimism about containing the pathogen contradicted a senior health official in his own administration and came as authorities said they had identified the first case in the US that was of unknown origin.

"I don't think it's inevitable. I think that there's a chance that it could get worse, a chance it could get fairly substantially worse, but nothing's inevitable," Trump told reporters at the White House.

The Centers for Disease Control and Prevention urged Americans Tuesday to be prepared to cancel mass gatherings and urged schools and businesses to develop teleworking plans, amid dire warnings that countries are not ready to contain an outbreak that has infected 80,000 people, mostly in China.

US President Donald Trump, flanked by Health and Human Services Secretary Alex Azar (R), US Vice President Mike Pence (L) and CDC Principal Deputy Director Anne Schuchat, holds a news conference on the COVID-19 outbreak US President Donald Trump, flanked by Health and Human Services Secretary Alex Azar (R), US Vice President Mike Pence (L) and CDC Principal Deputy Director Anne Schuchat, holds a news conference on the COVID-19 outbreak Photo: AFP / Andrew CABALLERO-REYNOLDS

"It's not so much a question of if this will happen anymore, but rather more a question of exactly when this will happen," said the CDC's Nancy Messonnier.

As of Wednesday, there were 60 cases in the United States -- 15 detected through the public health system, and 45 repatriated from abroad, either from a cruise ship off Japan or from Wuhan, the Chinese city where the first cases were identified.

The CDC later said the latest case caught in California "did not have relevant travel history or exposure to another known patient" -- meaning it could represent the first instance of "community spread" in the US, though this was not yet confirmed.

Community spread means spread of an illness for which the source of the infection is unknown.

US President Donald Trump has defended his administration's response to the novel coronavirus, lashing the media for spreading panic as he conducts an evening news conference on the epidemic US President Donald Trump has defended his administration's response to the novel coronavirus, lashing the media for spreading panic as he conducts an evening news conference on the epidemic Photo: AFP / Eric BARADAT

Foreign nationals traveling from China within the past two weeks are currently banned from entering the US, and Trump said more countries, including South Korea and Italy where the disease has rapidly spread, could also be added to the list.

"At the right time we may do it, right now it's not the right time," he said in response to a reporter's question.

"Italy is, you know, a deeper problem, and we're checking people coming in very, very strongly from those and at some point we may cut that off," he later added.

Foreign nationals traveling from China are currently banned from entering the US and more countries, including South Korea and Italy could also be added to the list Foreign nationals traveling from China are currently banned from entering the US and more countries, including South Korea and Italy could also be added to the list Photo: AFP / Mark RALSTON

On Wednesday night the State Department raised its travel advisory caution level for South Korea to the second-highest, now urging Americans to reconsider traveling there.

The president added that Vice President Mike Pence had been assigned to lead the charge in coordinating the government's response and reporting back to him, taking over the role from health secretary Alex Azar.

Trump's briefing came after US lawmakers accused the administration of downplaying the crisis and underfunding the response.

On Monday, the White House asked Congress to make at least $2.5 billion available for preparedness and response, including developing treatments and vaccines and buying equipment for a strategic national stockpile.

Senate Democratic leader Chuck Schumer said the figure was "too little and too late," proposing instead $8.5 billion. Trump later told reporters he would be happy to accept more money.

Trump also took to Twitter earlier in the day to insist his administration is "doing a GREAT job," blaming the media Wednesday for trying to stoke fear in a tweet in which he spelled the name of the virus incorrectly.

"Low Ratings Fake News MSDNC (Comcast) & @CNN are doing everything possible to make the Caronavirus look as bad as possible, including panicking markets, if possible," he tweeted, using a derogatory name for cable news network MSNBC that suggests it is connected to the Democratic National Committee

Wall Street stocks finished mostly lower Wednesday, stabilizing somewhat after a two-session rout amid growing worries over the economic impact of the coronavirus epidemic.

With cases being reported in more countries -- and lockdowns in nations including Austria, Italy and Spain -- traders are worried about the impact on the global economy.

In the US, officials have voiced fears for the supply of medical products because a high proportion of ingredients used to make medicine comes from China.

The Food and Drug Administration has also identified personal protective equipment, such as face masks, respirators and gowns, as being among products at risk of shortages.