The US National Guard has been activated in New York, California and Washington to aid in their response to the coronavirus pandemic.

After New York Governor Andrew Cuomo and officials across the US pressured the federal government for days to ramp up its response to a growing crisis in their respective states, Donald Trump has announced that he signed off on a major disaster declaration for New York, per the governor's request.

New York also will get support to build four temporary hospitals, after Governor Cuomo announced plans for those centres early on Sunday. The federal government also will be shipping "large quantities" of medical supplies as overwhelmed hospital systems brace for surges in Covid-19 patients.

Health workers across the US have sounded the alarm of dwindling supplies and shortages of hospital beds for several weeks as the spread of the virus threatened already-stressed health systems.

A hospital ship, the USS Mercy, will also be deployed to Los Angeles to expand the California's medical capacity.

FEMA is expected to supply four medical stations with 1,000 beds in New York, as well as eight medical stations with 2,000 beds in California.

The agency also is expected to support several medical stations with 1,000 beds in Washington.

That aid will be delivered within 48 hours, the president said.

The president announced the latest emergency response from the White House on Sunday after facing another day of criticism from state and local leaders, pressuring the president to lean on a war-time measure that would mandate the manufacturing of critically needed health supplies like ventilators and facemasks.

State leaders also say that states are left to compete with one another while the federal government has outbid states vying for those supplies after the president suggested last week that they fend for themselves.

On Sunday, FEMA's Peter Gaynor admitted that the president did not order any company to manufacture supplies after Mr Trump invoked the Defence Production Act earlier this week but has since withdrawn from the law.

White House economic advisor Peter Navarro said the administration instead is using "quiet leverage" to mobilise companies: "We're getting what we need without the heavy hand of government."

The president conflated the use of the law as a move to nationalise a supply-manufacturing industry: "We're a country not based on nationalizing our business. Call a person over in Venezuela ... How did nationalisation of their businesses work out? ... Not a good concept."

He said that 3M, General Motors and Hanes are voluntarily producing facemasks but did not offer specifics.

During his Sunday press conference, Mr Trump meandered from the administration's response to complaints about money he lost as president and false claims about a drug to combat the virus.

The briefing followed reports that the administration had eliminated an American public health position in China that intended to detect outbreaks in that country. That medical epidemiologist left her post in July, Reuters reported.

Mr Trump also failed to answer whether he would support a federal bailout that would benefit his businesses and instead said that "nobody said thank you" when he donated his presidential salary.

He repeated false claims that an anti-malarial drug will work on coronavirus and will begin testing the medicine on Tuesday, despite being refuted by health officials in his own administration.

Mr Trump said there was "very strong evidence" that it would work.

Dr Anthony Fauci, the top doctor leading the administration's response to the virus, has said that the "evidence" is only anecdotal, among many of the president's inflated claims that he has had to correct.

He did not appear at Sunday's press conference.