The White House has sent governors the outlines of a process by which Donald Trump intends to reopen parts of the country by classifying individual counties into three risk categories, contending "enhanced testing" is allowing his government to "isolate" the virus and stop its spread.

The president sent the letter the same week he has been pushing hard to get all or part of the United States "open for business" by 12 April.

He has said his goal is based on that being a "beautiful time," but has yet to described a scientific rationale to support his claim. So far, he has spoken purely in economic terms, saying at a Wednesday night White House virus briefing that the economy was booming before the bug crept onto US soil and began infecting tens of thousands. Mr Trump is eager to return the economy and US stock markets to their pre-coronavirus health as he seeks a second term.

"There is still a long battle ahead, but our efforts are already paying dividends. As we enhance protections against the virus, Americans across the country are hoping the day will soon arrive when they can resume their normal economic, social and religious lives," Mr Trump wrote to the state leaders. "My administration is working to publish new guidelines for state and local policymakers to use in making decisions about maintaining, increasing, or relaxing social distancing and other mitigation measures they have put in place."

"This is what we envision: 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," he wrote, promising "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 categorising counties as high-risk, medium-risk, or low-risk," Mr Trump said.

Major US stock indexes appeared initially undeterred by the letter. They remained in the green territory after the missive was released by the White House, continuing another day of gains after dropping substantially since the pandemic hit American soil.

The letter went out to state chiefs executive a day after a new Gallup poll found 60 per cent of Americans approve of the president's response to the sometimes-deadly virus. Of that group, 60 per cent of the key independents voting group approve as his overall approval rating again hit its record: 49 per cent.

Mr Trump is trying to capitalise on those new numbers while he is as popular as he has been since taking office in January 2017, eager to get the economy rolling again as tens of millions of Americans are breaking records by seeking unemployment benefits. State and local governments have shuttered non-essential business, hitting owners and workers in industries like the bar and restaurant sector hard as many rank and file employees have been laid off.