A new coronavirus patient in California could be the first person infected by another individual within the US.
On Thursday, the Centers for Disease Control and Prevention (CDC) said, "at this time, the patient's exposure is unknown".
"[But] it's possible this could be an instance of community spread of COVID-19, which would be the first time this has happened in the United States," it said.
As of Thursday, there were 60 cases in the US - 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.
It came as President Donald Trump said he is considering travel restrictions on Italy and South Korea.
"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," Mr Trump told reporters at the White House.
The CDC urged Americans 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.
U.S. President Donald Trump looks on after a news conference with Vice President Mike Pence. Source: Getty Images North America
"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.
More travel restrictions?
Foreign nationals traveling from China within the past two weeks are currently banned from entering the US, and Mr 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.
The State Department has raised its travel advisory caution level for South Korea to the second-highest, now urging Americans to reconsider traveling there.
Chinese citizens wear face masks as they check in to their Air China flight to Beijing, at Los Angeles International Airport. Source: AFP
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.
Extra funding
Mr Trump's briefing came after US lawmakers accused the administration of downplaying the crisis and underfunding the response.
Earlier this week, 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. Mr Trump later told reporters he would be happy to accept more money.
Mr 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, stabilising 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.
