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.

Mr Trump’s announcement late on Monday comes as the White House argues the worst of the pandemic is over and the country can 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 last four weeks, more than 20 million Americans have made jobless claims. That amounts to roughly as many jobs as employers had added over the previous decade.

The US has over 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?

It was not immediately clear who could be affected by Mr Trump’s decision or when it could come into force, and the White House has not commented.

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 border with Mexico. 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. 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.