US officials are demanding Donald Trump use war-time emergency powers to accelerate the production of critically needed medical supplies, after the president appeared to renege on plans to use a law that he already invoked and later said was in “high gear”.

Confusion about the White House timeline around its use of the Korean War-era Defence Production Act arrives as hospitals and health workers in cities and states across the US already are facing a shortage of ventilators, face masks, diagnostic testing supplies and other equipment in the wake of the coronavirus pandemic.

On Friday, the president said that rather than mandate companies to increase production of those supplies, the US is ”literally being besieged in a beautiful way by companies that want to do the work and help our country”.

The president said there were “millions of masks coming” and will be distributed “directly” to states.

Before his Friday press conference, Mr Trump spoke with Senate Minority Leader Chuck Schumer, who urged the president to rely on the Defence Production Act. According to a spokesman for the senator, the president “yelled to someone in his office to do it now” during the call.

But just two days earlier, Mr Trump said he had only invoked the act for a “worst-case scenario in the future” and that “hopefully there will be no need, but we are all in this TOGETHER!”

On Thursday, House Speaker Nancy Pelosi said the shortages “are harming our ability to fight the coronavirus epidemic, endangering frontline workers and making it harder to care for those who fall ill.”

She said: “The president must immediately use the powers of the Defence Production Act to mass produce and coordinate distribution of these critical supplies, before the need worsens and the shortages become even more dire. There is not a day to lose.”

The act was initially passed in 1950 at the outset of the Korean War under then-president Harry Truman for war-time metal production.

New York Mayor Bill de Blasio slammed the White House response as “weeks, if not months, behind the crisis” and claimed that his calls to administration officials in the Health Department and Veterans Affairs have been ignored.

He told MSNBC on Friday: “The federal government basically doesn’t exist at this moment.”

If the president doesn’t act in “a matter of days” and mobilise manufacturing as well as the US military, “a lot of people are going to die who don’t have to die” if the city’s health supplies continue to diminish within a few weeks.

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The mayor says he has repeatedly requested emergency supplies of ventilators and masks for not just health workers for police and other first responders but has been met with “silence” from the administration.

He also has demanded that the president send the military into the city to coordinate the response, after New York saw a surge of nearly 4,000 cases of Covid-19 while the hospital system faces a dramatic shortage of necessary supplies.

“Where is there a greater threat to America lives?” Mayor de Blasio said. “He’s not acting like the commander in chief. ... He should get the hell out the way and let the military do its job.”

The White House has been reluctant to lean on those war-time powers as the administration increasingly promises other federal intervention despite conservatives’ small-government ideology, conflicting with an administration response that has failed to find consistent messaging following the pandemic declaration.

Mr Trump told reporters on Thursday: “The federal government is not supposed to be out there buying vast amounts of items and then shipping. You know, we’re not a shipping clerk.”

But states have pushed back against the White House assertions that the federal government can’t put manufacturers to work, after claims from some states that the federal government has already denied sending requested equipment or falling significantly short on their end of the order.

New York Governor Andrew Cuomo said ”we literally have people in China shopping for ventilators” to beef up the state’s supply.

He said: “We’re going to need protective equipment and hospitals, we are going to need ventilators, and that is something that a state can’t do but the federal government can do.”

Federal health officials said the government will make 500m N95 respirators available over the next 18 months but it’s unclear when they’ll reach people who need them most.

The centers for disease control and prevention (CDC) has recommended the limited use of face masks, and suggested the use of surgical masks instead of N95s despite surgical masks not providing an effective filter to airborne viruses.