President Donald Trump has said his plan to ban immigration into the US will last for 60 days and apply only to those seeking permanent residence.
Mr Trump, who had announced the move in an ambiguous tweet, said the measure would protect Americans who had lost jobs because of the coronavirus.
The decision does not apply to workers entering on a temporary basis.
Critics say he is trying to distract attention away from his handling of the spread.
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 said that his executive order was being written and is expected to completed by White House lawyers on Wednesday for him to sign. The ban could be extended “much longer” depending on how the economy is doing, he said.
“As we all know, millions of Americans sacrificed their jobs in order to battle the virus and save the lives of our fellow citizens,” he said at a news conference.
The president said the government has a “solemn duty” to ensure Americans regain their jobs.
“It would be wrong and unjust for Americans laid off by the virus to be replaced with new immigrant labour flown in from abroad,” he said.
“We want to protect our US workers and I think as we move forward we will become more and more protective of them”.
The US has recorded more cases of coronavirus than anywhere else in the world at more than 820,000 cases, according to the Johns Hopkins University.
How much will this change?
The ban is watered-down in comparison to the president’s vow to suspend “all immigration to US” in a tweet late on Monday.
But while such a move would usually affect millions, travel bans to prevent the spread of Covid-19 has already led to a huge drop in people planning to enter the country.
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.
What are green cards?
In a typical year, nearly one million green cards are issued in the US, granting immigrants legal permanent residence and the opportunity to apply for American citizenship.
The majority of green cards – roughly 70% – go to those with relatives living in the US, according to a 2018 report from the US Senate.
For employment-based green cards, a common form of the residency status, roughly 80% are issued to those already in the country, shifting from a temporary visa to permanent residence.