In the letter, Trump said expanded testing capabilities in the country would enable the federal government to label "high-risk, medium-risk and low-risk" counties with differentiated social distancing requirements.
"Our expanded testing capabilities 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," the president wrote.
Based on the new guideline, of which the date of adoption is not yet known, governors and other state policymakers will be better able to decide on "maintaining, increasing or relaxing social distancing and other mitigation measures they have put in place," Trump said.
Deborah Birx, a member of the White House Coronavirus Task Force, said at a press briefing later in the day that what is vital for the stratification system is that people from a high-risk region should be prevented from traveling to a low-risk region.
"Part of this will be the need to have highly responsible behavior between counties," she said. "I think the American people can understand where the virus is, and where it isn't, and make sure that they're taking appropriate precautions."
The new guidance came as the United States now has the most confirmed cases in the world, reaching 85,840 as of 11:47 p.m. ET on Thursday (0347 GMT on Friday), according to the latest data from Johns Hopkins University. Deaths caused by the disease rose to 1,296 in the country.
As the 15-day social distancing strategy he announced last week nears the end, Trump has in recent days floated the idea of loosening the restrictive measures.
"I gave it two weeks," he said Tuesday during a virtual town hall hosted by Fox News aired from the White House Rose Garden. "I guess, by Monday or Tuesday, it's about two weeks. And we will assess at that time."
The president said he wants the United States to be "opened up" by Easter, which falls on April 12 this year, despite warnings from both health experts and state governors that doing so risks failing to contain the escalating spread of the coronavirus.
Facing a critical shortage of ventilators in his state, New York Governor Andrew Cuomo was visibly outraged by the federal government's response to the crisis. "The president says it's a war," he said at a news briefing on Tuesday. "Well, then, act like it's a war!"
New York State now tops all other states in both confirmed cases and fatalities. Cuomo said the state needs 30,000 additional ventilators to cope with the dire situation, but Vice President Mike Pence told reporters at the White House on Tuesday that 4,000 ventilators will be sent to New York.
Trump reiterated Thursday his desire to start opening the country in a piecemeal approach.
"I think we can start by opening up certain parts of the country: you know, the farm belt, certain parts of the Midwest, other places," he told Fox News' Sean Hannity in an interview in the evening. "I think we can open up sections, quadrants, and then just keep them going until the whole country is opened up."
He also questioned the need for 30,000 ventilators in New York. "I don't believe you need 40,000 or 30,000 ventilators. You know, you go into major hospitals, sometimes they'll have two ventilators. And now all of a sudden they're saying, 'can we order 30,000 ventilators?'"
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