More than 1,700 Americans have now tested positive for the virus, and 41 have died.
Speaking in the Rose Garden of the White House, Mr Trump said declaring an emergency would allow the health and human services secretary, Alex Azar, to waive “certain laws and regulations” to give healthcare operators more flexibility.
Mr Trump said there would be 500,000 extra tests available early next week. The administration has been criticised for its slow response to making tests available. However, the president stressed that authorities do not want people who do not need tests to take one.
He said the national emergency – which he referred to as "two very big words" – would not be needed for long.
Google is working on a website that will help people determine whether they need a test and help them get access to one.
Asked about whether he himself would take a test, having been exposed to people who have tested positive for the coronavirus, Mr Trump said that he had no coronavirus symptoms himself. Later, when asked again, he said: "I will most likely get tested."
He said that his exemption of the United Kingdom from European countries facing travel restrictions to the US may have to be rethought after the number of confirmed cases there increased "precipitously".
He said other countries could be added or removed to the list.
Asked if he took any responsibility for his administration's slow response to making tests available, the president said: "No, I don't take responsibility at all." He said the "rules and regulations" he inherited were not designed for an outbreak like coronavirus.
Pressed on dismantling the pandemic response team after coming to power in 2017, he said: "I think it's a nasty question ... We've saved thousands of lives. And when you say 'me', I didn't do it, we have a group of people ... I don't know anything about it."
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