US President Donald Trump has announced the suspension of all travel to the United States from the majority of European nations for 30 days, in a bid to combat the spread of the coronavirus.
The travel restrictions will apply to the 26 members of the Schengen border-free area of the EU, which does not include Ireland or the UK.
Mr Trump said this would go into effect from "midnight Friday".
In a primetime address from the Oval Office, Mr Trump sought to rebuff critics who say his leadership has been lacking during the crisis, insisting that "the virus will not have a chance".
"This is the most aggressive and comprehensive effort to confront a foreign virus in modern history," he said.
He announced a series of measures designed to ease the financial burden for people having to take weeks off from work while on quarantine.
But his most far-reaching announcement was the halt on European travel.
The EU will assess the travel ban situation later today, European Council President Charles Michel said, adding: "Economic disruption must be avoided."
"Europe is taking all necessary measures to contain the spread of the COVID-19 virus, limit the number of affected people and support research," he said.
A statement on the White House website said: "For purposes of this proclamation, the Schengen Area comprises 26 European states: Austria, Belgium, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Italy, Latvia, Liechtenstein, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Slovakia, Slovenia, Spain, Sweden, and Switzerland."
"I'm confident that by ... continuing to take these tough measures, we will significantly reduce the threat to our citizens and we'll ultimately and expeditiously defeat this virus," he said.
Mr Trump called on elderly people to avoid non-essential travel within the US.
The White House said Mr Trump would be cancelling a trip to Nevada and Colorado out of "caution".
Around 80% of cases of Covid-19 will be a mild to moderate illness, close to 14% have severe disease and around 6% are critical.
Generally, you need to be 15 minutes or more in the vicinity of an infected person, within 1-2 metres, to be considered at-risk or a close contact.
In his address, Mr Trump put a harsh accent on Europe's role in the spread of the virus, which began in China and has now travelled around much of the world, bringing sometimes fatal flu-like symptoms.
"The European Union failed to take the same precautions and restrict travel from China and other hotspots" as the US, Mr Trump said.
"As a result, a large number of new clusters in the United States were seeded by travellers from Europe."
About 4,500 people have died so far worldwide.
As of yesterday, there were more than 1,100 confirmed infections in the US and 32 deaths, according to a tally by Johns Hopkins University.
Numbers are expected to skyrocket in the United States once more testing is done.
Anthony Fauci, the director of the National Institute of Allergy and Infectious Diseases, said that COVID-19 is estimated to be 10 times more deadly than flu and that without aggressive containment and mitigation, "many millions" could potentially contract the virus.
"The bottom line: it's going to get worse," Mr Fauci said.
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Mr Trump announced short-term tax deferments and called on Congress to enact "immediate payroll tax relief".
He also said he was asking for another $50bn in funding for small business loans to help those stricken by the economic disruption.
However, there was no declaration of a national emergency which would allow the government instant access to extra funding.
It was not clear whether Mr Trump's plans would calm markets, which have panicked over the mounting impact on supply lines, consumer demand and investor flight to safety.
"This is not a financial crisis," Mr Trump said. "This is just a temporary moment of time that we will overcome together as a nation."
Multiple cultural and sporting events have been cancelled across the United States, schools and universities are closing, and companies are scrambling to face the probability of sending many employees home on sick leave or self-quarantine.
Even the US presidential election faces disruption, with Democratic candidates Joe Biden and Bernie Sanders cancelling rallies and questions mounting over the future of Mr Trump's own usually frequent rallies.
For Mr Trump, steadying market jitters is a must if he is to maintain his reelection message of delivering a booming economy.
At a White House meeting with US bank leaders, Mr Trump said "we'll be doing a lot of additional work with small businesses ... many billions of dollars".
Treasury Secretary Steven Mnuchin said earlier that the White House was "looking at providing substantial relief to certain taxpayers and small businesses who will be able to get extensions on their taxes".
"We think we can provide over $200bn of liquidity into the economy by delaying certain tax payments," he said.
Democratic leaders are cool on the idea of tax cuts and made a competing proposal for compensating workers' lost wages and sick leave, as well as providing loan forgiveness for individuals.
The Democratic proposal also called for disaster loans and grants for small businesses, emergency housing assistance, and loosened requirements for food assistance including continuing to serve meals to children even if schools are closed.