Managing dual health and economic crises, President Donald Trump announced on Wednesday that he will invoke a federal provision that allows the United States government to marshal the private sector in response to the coronavirus pandemic.

Trump, appearing in the White House briefing room for the third day in a row, said he would sign the Defense Production Act "in case we need it" as the government bolsters resources for an expected surge in cases of the virus.

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Trump also said he will expand the nation's testing capacity and deploy a navy hospital ship to New York City, which is rapidly becoming the epicentre of a pandemic that has rattled the US economy and rewritten the rules of American society. A second ship will be deployed to the West Coast.

The president also said the Housing and Urban Development Department will suspend foreclosures and evictions through April as a growing number of Americans face losing jobs and missing rent and mortgage payments.

The steps came on a fast-moving day of developments.

US-Canada border to partially close

Trump and Canadian Prime Minister Justin Trudeau jointly announced that the US-Canada border would be closed, except for essential personnel and for trade, as the nations try to reduce the spread of the virus afflicting people in both countries.

The administration has told Americans to avoid groups of more than 10 and the elderly to stay home while a pointed reminder was given to millennials to follow the guidelines and avoid social gatherings. Trump likened the effort to the measures taken during World War II and said it would require national "sacrifice".

Trump has come under fire for initially downplaying the threat of the virus, which has now killed more than 100 across the US.

Moments before telling the country that the US and Canada had decided to temporarily close the world's longest border to non-essential traffic, Trump was tweeting about his approval rating and bashing the news media that his administration is relying on to share its warnings about the virus.

"I always treated the Chinese Virus very seriously, and have done a very good job from the beginning," Trump tweeted, despite his record of playing down the threat for weeks. "The Fake News new narrative is disgraceful & false!"

Trump has also caused outrage for repeatedly calling the virus the "Chinese Virus" - a label rights groups and others have said "fuel bigotry".

As he needled Democrats, Trump tried to assure those who are now out of work as hotels, bars, restaurants and other gathering spots close that, "money will soon be coming to you. The onslaught of the Chinese Virus is not your fault! Will be stronger than ever!"

Asked on Tuesday whether he would consider laying off the insults during a national emergency, Trump said he would continue to respond "if they're not going to play fair".

"I want it to be bipartisan and nobody's going to be better than me," he added. "But when they attack me or the people - these incredible people behind me - I'm not going to let them get away with that. I can't do that."

Since the virus has spread, Trump and his top aides have stepped up their criticism of China, noting consistently that the outbreak was first reported in the Chinese city of Wuhan in late 2019.

For most people, the new coronavirus causes only mild or moderate symptoms, such as fever and cough. For some, especially older adults and people with existing health problems, it can cause more severe illness, like pneumonia.

The vast majority of people recover from the new virus. According to the World Health Organization, people with mild illness recover in about two weeks, while those with more severe illness may take three to six weeks to recover.