President Donald Trump has said he will sign an executive order to temporarily suspend all immigration to the US because of the coronavirus.
On Twitter, he cited "the attack from the invisible enemy", as he calls the virus, and the need to protect the jobs of Americans, but did not give details.
It was not clear what programmes might be affected and whether the president would be able to carry out the order.
Critics say the government is using the pandemic to crack down on immigration.
Immigration has traditionally been a strong campaigning theme for Mr Trump, but has taken a back seat during the pandemic and in the lead-up to the November election.
In light of the attack from the Invisible Enemy, as well as the need to protect the jobs of our GREAT American Citizens, I will be signing an Executive Order to temporarily suspend immigration into the United States!— Donald J. Trump (@realDonaldTrump) April 21, 2020
Mr Trump's announcement late on Monday came as the White House argued that the worst of the pandemic was over and the country could begin reopening. The restrictions on people's movement, implemented by many states to curb the spread of the virus, have paralysed parts of the economy.
Over the past four weeks, more than 20 million Americans have registered for unemployment benefits. That amounts to roughly as many jobs as employers had added over the previous decade.
The US has more than 787,000 confirmed cases of Covid-19 and more than 42,000 deaths, according to a tally by Johns Hopkins University, which is tracking the pandemic globally.
How can the proposals be implemented?
A Trump campaign statement on Tuesday said the move had two functions - to prevent competition for jobs that could add to dole queues and depress wages, and stop imported infections.
It was not immediately clear who could be affected by Mr Trump's announcement or when such a move could come into force.
According to the New York Times, citing several people familiar with the plan, a formal order temporarily barring the provision of new green cards and work visas could be one way of implementing the measure; the administration would no longer approve any applications from foreigners to live and work in the US for an undetermined period of time.
Last month, the US suspended almost all visa processing, including for immigrants, because of the pandemic.
The US has already agreed with both Canada and Mexico to extend border restrictions on non-essential travel until at least mid-May.
Travel has also been sharply restricted from hard-hit European countries and China, though people with temporary work visas, students and business travellers are exempted.
On Monday, the US said it would continue to expel migrants it encounters along the border with Mexico for at least another month.
In recent weeks, emergency powers have been used to expel thousands of undocumented migrants on the US southern border. The public health measure lets officials override immigration laws, expediting removal processes.
Last year, just over one million people were granted lawful permanent resident status in the US, according to the Department of Homeland Security. The top countries of origin were Mexico, China, India, the Dominican Republic, the Philippines and Cuba.
More than half of those, though, were cases of "adjusted status from within the US" - meaning they were already there - and only 459,000 arrived from abroad. The latter group would be the ones presumably affected by an immigration ban.
When it comes to refugees, there were 30,000 people admitted into the US in 2019, most of them from Congo, Myanmar, Ukraine, Eritrea, Afghanistan and Syria.
What are the reasons behind the move?
Donald Trump's efforts at governing by social media should always be taken with a sizable grain of salt. His track record on following through on Twitter directives is decidedly mixed. The details of his temporary ban on all immigration, announced a few hours before midnight on Monday, will shed considerable light on the breadth - and legality - of his actions.
Still, it is no secret that the president, and several key advisers, have long viewed immigration not as a benefit to the nation, but as a drain. And the text of his tweet, that the move is necessary not only to protect the nation's health but also "the jobs of its great American citizens", only emphasises this.
There is little doubt the proposal, in whatever form it takes, will be vigorously opposed by pro-immigration groups, some business interests and the president's ideological adversaries. That is probably just fine with a man who loves drawing political battle lines and goading his opponents whenever possible.
Four years ago, the president campaigned on an aggressive anti-immigration platform, including a total, if temporary, ban on all Muslims entering the country. Now, with an uphill re-election fight looming, he has found a similarly combative measure to champion.
What has the reaction been?
Criticising the ban, California Democratic Senator Kamala Harris said Mr Trump was using the outbreak to push his anti-immigrant stance.
Trump failed to take this crisis seriously from day 1. His abandonment of his role as president has cost lives. And now, he's shamelessly politicizing this pandemic to double down on his anti-immigrant agenda.— Kamala Harris (@SenKamalaHarris) April 21, 2020
Enough, Mr. President. The American people are fed up. https://t.co/pT5edXBIT5
Former Democratic presidential candidate Amy Klobuchar also condemned the decision.
As our country battles the pandemic, as workers put their lives on the line, the President attacks immigrants & blames others for his own failures. The order I’d like to see tonight? Testing. Instead he twists “the buck stops here” into this: “the buck stops anywhere but here.”— Amy Klobuchar (@amyklobuchar) April 21, 2020
Just hours after Mr Trump's announcement, former Vice-President Joe Biden wrote on Twitter that the president should "do his job", and stop seeking ways to redirect blame - an apparent reference to the immigration pause. The presumptive Democratic nominee was echoed by Senate Minority Leader Chuck Schumer on Tuesday who dismissed the move as "another diversion".
But the announcement was welcomed on the Republican side. Congressman Paul Gosar, who represents Arizona, applauded the move:
What's the latest in the US?
Earlier on Monday, Democratic governors asked the White House to urge Americans to heed stay-at-home orders amid anti-lockdown protests stoked by the president.
Mr Trump has been accused of inciting insurrection after championing the demonstrators, while telling governors they were in charge.
He has expressed his support of the protesters in recent days, even as state governors say they are following White House guidance for safely reopening in phases.
A number of southern states in the US are in the process of easing virus-related restrictions.
South Carolina has allowed some retailers, including department stores, to re-open, while most businesses in Tennessee will re-open on 1 May.
Georgia's governor has said residents would be allowed to visit gyms, hairdressers and tattooists from Friday, followed by restaurants and cinemas on Monday, as long as those businesses operated within social distancing guidelines.
The Republican governors of all three states say social distancing measures will remain in place.
The WHO has previously warned about easing restrictions too early to avoid seeing a resurgence of infections.
Mr Trump - who faces an election in November - last week tweeted in all capital letters for several states to be liberated. At Sunday's coronavirus briefing, he said those protesting against their governors' social distancing measures were "great people".
"Their life was taken away from them," he said. "These people love our country, they want to get back to work."