Diana Torres, a nurse at Mount Sinai in New York City, is in a fearful mood as New York records its highest number of coronavirus-related deaths in a single day.

“I’m paranoid, scared to walk out of their room.”

Patients “look fine, feel fine, then you turn around and they’re unresponsive,” Ms Torres said.

The number of coronavirus cases in New York state alone approached 150,000 on Wednesday (local time), the highest in the US, even as authorities warned the state’s official death tally may understate the true number.

Veteran doctors and nurses are expressing shock at the speed with which patients are declining and dying.

It isn’t just elderly or patients with underlying health conditions who appear to be fine one minute and at death’s door the next. It can happen for the young and healthy, too.

• Related: Americans told to brace for ‘hardest week’

“Every number is a face, ” said New York Governor Andrew Cuomo, who ordered flags flown at half-staff across New York in recognition of the toll.

“This virus attacked the vulnerable and attacked the weak and it’s our job as a society to protect the vulnerable.”

New York has become the epicentre of the pandemic in the United States where the virus has infected more than 415,000 people.

Officials said a recent surge in the number of people dying at home suggests the most populous US city may be undercounting how many have died of COVID-19, the respiratory illness caused by the pathogen.

“I think that’s a very real possibility,” Mr Cuomo told his daily news briefing.

Members of The National Guard walk through Manhattan. Photo: Getty

Mr Cuomo said 779 people died in the past day in his state. New Jersey Governor Phil Murphy said 275 had died there. Both totals exceeded one-day records reported just a day earlier.

Mr Cuomo said the death count would continue at the current level or increase in the coming days as critically ill patients, who have been hospitalised for more than a week and on ventilator machines to assist in breathing, die.

Tragically, New York lost an additional 779 people yesterday to COVID-19.

They are more than just a statistic.

Every number is a face. Every number is a family.

We mourn these New Yorkers deeply.

— Andrew Cuomo (@NYGovCuomo) April 8, 2020

California, like New York, had one of its highest single-day death tolls with 68 people dying of COVID-19 in the past 24 hours, Governor Gavin Newsom said.

Louisiana Governor John Edwards said the number of new coronavirus cases reported in the past 24 hours – 746 – was lower than recent days. Louisiana had been one of the nation’s hot spots for the virus.

“We do believe we are beginning to see the flattening of the curve,” he said.

Meanwhile, President Donald Trump said he would like to reopen the US economy with a “big bang”, but that the death toll from the coronavirus needs to be on the down slope before that can happen.

-with agencies