The chief infectious disease expert leading the US response to the coronavirus says he "doesn't understand" why every state hasn't issued stay-at-home orders, creating patchwork quarantine efforts across the country.

His frustration contradicts instruction from White House officials who have praised stringent measures in hard-hit states and cities but insist on respecting "statehood" in the face of the crisis.

Dr Anthony Fauci told CNN: "You know, the tension between federally mandated versus states' rights to do what they want is something I don't want to get into ... But if you look at what's going on in this country, I just don't understand why we're not doing that."

More than 30 states and Washington DC have ordered residents to stay at home, with just five states without any stay-at-home orders nor mandated closures of nonessential businesses. Arkansas, Iowa, Nebraska, North Dakota and South Dakota don't have those measures in place on state or local levels.

Asked why he hasn't considered a national order, Donald Trump said "because states are different" and that "some states don't have much of a problem."

At a White House briefing on 1 April, he said: "They don't have thousands of people that are positive or thousands of people that think they might have it, or hundreds of people in some cases." (This week, South Dakota became the last state to report more than 100 cases.)

Asked whether every state would have a stay at home order, US Surgeon General Jerome Adams told NBC that the White House "guidelines" are essentially their own "national stay-at-home order."

He said that the administration wants "nationally people to understand the importance of social distancing" and is leaving those guidelines to states to determine their plans.

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Mike Pence told CNN: "At the present moment, we truly do believe that the strong actions taken in places like California and Washington and New York and New Jersey are appropriate ... We fully support those efforts."

The vice president said the administration is "going to continue to bring the president the best recommendations based on real-time data and science for what every state, what every community should be doing" but is aiming to "reopen" the country by early June and "put America back to work."

He said: "We could well have the coronavirus largely behind us."

California Governor Gavin Newsom, who was the first governor to issue a statewide measure after several Bay Area counties issued shelter-in-place orders, told CNN: "What more evidence do you need? If you think it's not going to happen to you, there are many proof points all across this country -- for that matter, around the rest of the world."

Experts said that regardless of whether states have comprehensive and consistent quarantine measures, the damage is likely done. The social distancing measures should have been in place weeks ago for the US to begin seeing a "flatten the curve" effect, they say.