Donald Trump is extending a ban on travel between the US and European countries to include the UK and Ireland as part of the ramped up efforts to curtail the spread of the coronavirus.
Mr Trump, who the White House revealed had tested negative for the virus, also said he was considering domestic travel restrictions "specifically from certain areas".
"If you don't have to travel, I wouldn't do it," the president told reporters.
Mike Pence, the US vice-president, said the ban on travel from the UK and Ireland to the US would go into place from midnight Monday night eastern standard time.
"The president has made a decision to suspend all travel to the UK and Ireland," Mr Pence said. "All of our health experts presented information . . . made a unanimous recommendation to the president that we suspend all travel from the UK and Ireland."
"Americans in the UK and Ireland can come home," he added. "Legal residents can come home . . . they will be funnelled through specific airports and process."
Chad Wolf, the acting US secretary of homeland security, said the ban would apply to foreign nationals who had been in the UK or Ireland in the past two weeks. Mr Wolf said that US citizens and legal residents returning to the US from the UK or Ireland would be screened and asked to self-quarantine for 14 days upon arrival back in the US.
Downing Street said on Saturday that Mr Trump had spoken with UK prime minister Boris Johnson.
"They discussed the coronavirus pandemic and the action being taken to stop the spread of the virus," a Number 10 spokesperson said. "The prime minister set out the science-led approach the UK is taking...[Mr Trump and Mr Johnson] agreed on the importance of international coordination to accelerate progress on the development of a vaccine and to prevent economic disruption for our citizens."
Mr Trump told reporters at the White House on Saturday that he had been tested for Covid-19 on Friday night, just hours after the White House published a memo from the president's physician saying the 73-year-old was at "low risk for transmission" and did not need to be quarantined or tested.
On Saturday night, the House published a second memo from the president's doctor, saying that his test came back negative and he remained symptom-free.
Mr Trump hosted a Brazilian presidential delegation at his Mar-a-Lago resort in Florida last week. Multiple members of the delegation have since tested positive for the novel coronavirus. Last month, the president attended the Conservative Political Action Conference, where one attendee later tested positive for the virus.
The president has also spent time with Ronna McDaniel, the chairwoman of the Republican National Committee, who was tested for Covid-19 after exhibiting symptoms on Friday night and is awaiting her results.
On Wednesday, the White House announced measures banning visitors from European countries in the Schengen Area, which does not include the UK and Ireland but includes non-EU states such as Iceland and Switzerland.
The US clampdown comes as many countries take strict measures to prevent the spread of Covid-19. Spanish prime minister Pedro Sánchez announced a nationwide lockdown on Saturday, meaning people will only be able to leave their homes for essential tasks like buying food or attending medical appointments.
In France, the government ordered the closure of all cafés, restaurants, non-essential shops and places of entertainment from midnight on Saturday. Israel has also set out plans to shut restaurants, cafes, cinemas and other non-essential businesses, while banks, petrol stations and supermarkets will remain open.
The expansion of the travel restrictions further undermines the reeling airline industry. The three largest US carriers said yesterday they were seeking unspecified assistance from the federal government. Delta Air Lines plans to cut 40 per cent of its scheduled flights and park 300 aircraft, while chief executive Ed Bastian foregoes his salary for six months.
United Airlines derives the highest percentage of its revenue from transatlantic travel, at 17 per cent, according to a March 13 note from Morningstar analyst Brian Bernard.
But all three of the top US carriers generate the bulk of their revenue from domestic travel, and the move to restrict travel to and from US destination draws Southwest Airlines into the tailspin as well. So far the Dallas airline has avoided cutting capacity because it serves no routes to Europe or Asia, the areas most affected by Covid-19 so far.
"Southwest is aware of the comments made by the president and vice-president of the United States, and we will be ready to fully comply with any mandate delivered," an airline spokeswoman said.
On Friday, Mr Trump used a press conference in the White House Rose Garden to declare a US national emergency to free up more federal funds to fight the disease. The White House also struck an agreement late Friday with congressional Democrats on an economic stimulus package that includes two weeks of paid sick leave for workers afflicted by coronavirus.
The legislation passed the House of Representatives with overwhelming bipartisan support, and will be considered by the Senate last week. Congress earlier this month approved an $8.3bn emergency aid package to help states and local governments tackle coronavirus.
Steven Mnuchin, the US Treasury secretary, has likened the economic measures to a nine-inning baseball game, saying emergency package was the first inning and the deal struck on Friday was the second.
Nancy Pelosi, the Democratic speaker of the House who spent much of the week negotiating with Mr Mnuchin, said Friday that Democrats would soon begin work on a third piece of legislation intended to provide fiscal stimulus.
US stocks moved sharply higher during Mr Trump's Rose Garden press conference on Friday to end the day up 9.3 per cent, after suffering their sharpest decline since the 1987 crash on Thursday. The president told reporters on Saturday that he was "honoured" to see the market movements on Friday afternoon, saying they had set an "all-time record" and he should consider hosting a press conference every day, or "five times a day".
The president also praised the "very, very bipartisan" legislative deal struck between Mr Mnuchin and Ms Pelosi, which he said would "provide strong support for American families and communities".
But Mr Trump once again attacked the Federal Reserve and its chairman, Jay Powell, on Saturday, saying: "We hope the Federal Reserve will finally get on board and do what they should do. They should be much more proactive."
The president said other central banks were "taking much more aggressive action than our Fed, for the most part", adding that he had the "right" to remove Mr Powell from his post.
"He has so far made a lot of bad decisions in my opinion," Mr Trump said. "I am not happy with the Fed because I think they are following, not leading, we should be leading." The Fed cut interest rates earlier this month by a half percentage point, but the president has called for even lower rates and a refinancing of US government debt.
"We have some tremendous opportunities right now but Jerome Powell is not making it easy," Mr Trump said.
Addtional reporting by Daniel Dombey in Madrid, Victor Mallet in Paris and Mehul Srivastava in Tel Aviv