Donald Trump has said he is considering a quarantine for the coronavirus hotspots of New York, New Jersey and Connecticut despite it being unclear whether he has the power to order state residents to stay put.

The US president told reporters at the White House that he had spoken with Republican governor Ron DeSantis of Florida, among others, and that “a lot of the states that are infected but don’t have a big problem, they’ve asked me if I’ll look at it so we’re going to look at it”.

But New York governor Andrew Cuomo, who criticised the federal government’s response as his state became the country’s virus epicentre, said the issue had not come up in a conversation he had with Mr Trump earlier on Saturday.

“I don’t even know what that means,” the Democrat said at a briefing in New York. “I don’t know how that could be legally enforceable, and from a medical point of view, I don’t know what you would be accomplishing. I don’t like the sound of it.”

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Governor Andrew Cuomo criticised Mr Trump’s suggestion (Darren McGee/Office of Governor Andrew M. Cuomo/AP)

The federal government is empowered under the law to take measures to prevent the spread of communicable diseases between states, but it is not clear that means Mr Trump can ban people from leaving their state. It has never been tested in the modern era — and in rare cases when any quarantine was challenged, the courts generally sided with public health officials.

Courts have ruled consistently for years that the authority to order quarantines inside states rests almost entirely with the states, under provisions in the Constitution ceding power not explicitly delegated to the federal government to states.

The federal government, though, would have power under constitutional clauses regulating commerce to quarantine international travellers or those travelling state to state who might be carriers of deadly diseases.

“It is entirely unprecedented that governors or the president would prevent people from travelling from one state to another during an infectious disease outbreak,” said Lawrence Gostin, a Georgetown University law professor and public health specialist who questioned Mr Trump’s ability to order a quarantine on states.

But as Trump travelled to Norfolk, Virginia, to see off a US Navy medical ship setting sail for New York City to help with the pandemic response, he tweeted: “I am giving consideration to a QUARANTINE of developing “hot spots”, New York, New Jersey, and Connecticut. A decision will be made, one way or another, shortly.”

He may not need to order a legally justifiable quarantine.

One idea under consideration would be to tell residents of those areas to isolate themselves and not travel for two weeks, just as the Centres for Disease Control and Prevention have instructed anyone who recently left New York to self-quarantine for 14 days, according to one person familiar with the negotiations.

The measure would not necessarily come with any legal justification or penalty, just the hope that people would comply to try to contain the virus spread.

The governors of Florida, Maryland, South Carolina and Texas have ordered people arriving from the New York area to self-quarantine for at least 14 days upon arrival.

In a more dramatic step, Rhode Island police have begun pulling over drivers with New York plates so that the National Guard can collect contact information and inform them of a mandatory, 14-day quarantine.

Mr Trump said the idea of isolating many in the trio of Democratic strongholds in the north east was pushed by Mr DeSantis, one of the president’s most outspoken supporters. It came a day after Mr Trump made clear he wanted governors to be grateful when asking for federal support for the pandemic.

Mr Trump said people “go to Florida and a lot of people don’t want that. So we’ll see what happens”. He later clarified it would not affect lorry drivers or people transiting through, and would not affect trade.

“We’ll be announcing that one way or the other fairly soon,” he said.

Florida is a perennial swing state, and one Mr Trump must win come November — plus he recently moved his residence from New York to Florida. It also has a population of 21 million with a large percentage of old people, who are particularly vulnerable to the virus.

The US leads the world in reported cases with more than 115,000. There were roughly 1,900 deaths recorded by Saturday.

New Jersey governor Phil Murphy, a Democrat, said he did not talk about quarantining the tri-state area in his recent conversation with Mr Trump, and learned of the president’s comments as he walked into Saturday’s daily briefing.

“Until further notified we’re going to keep doing exactly what we’re doing, because we believe the data and the facts are on our side in terms of this aggressive, as aggressive as any American state right now, in terms of social distancing and flattening the curve,” he said.

In Norfolk, Mr Trump saw off the ship with a speech before only a few people and then watched as the USS Comfort slowly made its way out of port. The 1,000-bed hospital ship had been undergoing planned maintenance but was rushed back into service to aid the city.

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President Donald Trump waves as the US Navy hospital ship USNS Comfort pulls away from the pier (Patrick Semansky/AP)

It is scheduled to arrive on Monday at a Manhattan pier a week after its sister ship, the USNS Mercy, arrived in Los Angeles to perform a similar duty on the West Coast.

“We will stop at nothing to protect the health of New Yorkers and the health of the people of our country in their hour of need,” Mr Trump said.

The ship has 12 operating rooms as well as radiology suites and a CT scanner. It also has ICU beds, a lab and a pharmacy. The 1,100 or so medical staff on board are mostly active duty service members from the US Navy, and some reservists.