President Donald Trump on Thursday told governors the US has to "get smart" and ease coronavirus restrictions to reopen the economy.

"We have to open up our country, I'm sorry," Trump said on a phone call with governors.

This came as the president sent a letter to governors alerting them that the federal government was working on new coronavirus guidelines that would label counties based on their risk levels.

Public-health experts have said prematurely lifting coronavirus restrictions could exacerbate the pandemic.

The US has also lagged behind much of the world in terms of testing for the virus, which means it doesn't have a full picture of the scale of the outbreak within its borders.

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The US government is working to develop new guidelines for the coronavirus based on risk levels in counties across the country, President Donald Trump told governors in a letter on Thursday.

The plan would label counties as "high-risk, medium-risk, or low-risk."

"This is what we envision: Our expanded testing capacities will quickly enable us to publish criteria, developed in close coordination with the Nation's public health officials and scientists, to help classify counties with respect to continued risks posed by the virus," Trump said in the letter. "This will incorporate robust surveillance testing, which allows us to monitor the spread of the virus throughout the country. Under these data-driven criteria, we will suggest guidelines categorizing counties as high-risk, medium-risk, or low-risk."

Trump has grown impatient with the negative economic impact of restrictions put in place to quell the spread of the coronavirus pandemic in the US and pushed in recent days for the stringent measures to be scaled back. Earlier this week, Trump said he wanted the US opened up and "just raring to go" by Easter, which is April 12.

"We all have to get smart," Trump said on a phone call with governors on Thursday, a recording of which was obtained by The Associated Press. "We have to open up our country, I'm sorry."

During the call, Washington Gov. Jay Inslee challenged Trump after the president said his administration was ready to "back up" states in crisis because of the pandemic, according to The Washington Post. "We don't need a backup. We need a Tom Brady," Inslee said to Trump, referencing the six-time Super Bowl-winning quarterback.

The White House did not immediately respond to a request for comment from Insider. But at the daily White House press briefing later on Thursday, Trump confirmed Inslee referenced Brady but said the Washington governor meant it in a positive way.

The president has limited power in terms of forcing states to lift restrictions put in place to stop the spread of the coronavirus, largely because of the 10th Amendment of the US Constitution. And governors in hard-hit states have signaled they do not want to risk lives by prematurely removing measures such as shuttering nonessential businesses and ordering people to shelter in place.

"No American is going to say, 'Accelerate the economy at the cost of human life.' Because no American is going to say how much a life is worth," New York Gov. Andrew Cuomo said Tuesday during a press conference on the coronavirus. New York is the center of the coronavirus pandemic in the US, with over 37,000 confirmed cases in the state as of Thursday.

Public-health experts have also said that attempting to rush the US back to normal could exacerbate the impact of the pandemic across the country.

"You've got to understand that you don't make the timeline; the virus makes the timeline," Dr. Anthony Fauci, the nation's top expert on infectious diseases, told CNN on Wednesday.

The US has struggled to employ a robust system of testing for the virus because of early stumbles by the federal government. Many other countries are well ahead of the US in terms of testing rates, though Trump on Wednesday falsely said the US had tested more than any other country.

Without a robust system of testing, the US does not have a full picture of the scale of the coronavirus outbreak within its borders. This means there's a risk of the virus spreading even more if states lift guidelines that are meant to keep people indoors and reduce the risk of exposing themselves or others while the US ramps up testing. There was also no mention in Trump's letter to governors of restricting travel between high-risk and low-risk counties.

The World Health Organization on Wednesday said measures such as social distancing were "buying time" but added that countries needed to pair such restrictions with a comprehensive system of testing for the virus to defeat it.

As of Thursday, the US, which has a population of about 327 million, had tested a little more than 432,000 people for COVID-19, the disease caused by the novel coronavirus, according to the COVID Tracking Project. Comparatively, South Korea, which has a population of about 51 million, had completed 357,896 tests as of Wednesday, according to reports from the Korea Centers for Disease Control and Prevention.

As of Thursday, the total number of confirmed cases in the US had risen to more than 75,000, and at least 1,000 people in the US had died.