US President Donald Trump has signed an executive order suspending the approval of some immigration green cards for 60 days.

Mr Trump has said the aim is to keep foreigners out of local jobs during the coronavirus crisis.

"In order to protect our great American workers, I've just signed an executive order temporarily suspending immigration into the United States.

"This will ensure that unemployed Americans of all backgrounds will be first in line for jobs as our economy reopens," he told a press conference.

The 60-day suspension hits some of the people applying for permanent residency, or green cards.

The order exempts several categories of green card applicants, including people working in healthcare and immediate relatives of US citizens.

Seasonal workers, such as immigrants vital to US farm labour, are still allowed to enter.

Mr Trump said he is seeking to shield the job market for US citizens at a time when the previously booming economy has gone off a cliff, with 22 million joining the unemployment ranks due to the coronavirus-related shutdown.

"Without intervention, the United States faces a potentially protracted economic recovery with persistently high unemployment," the presidential order read.

With more than 46,000 deaths and 825,000 coronavirus infections, the United States is the world's hardest-hit country, and healthcare workers in hotspots such as New York have struggled to cope.

As the worst-hit areas report a decline in new cases and some states look to reopen their economies, mr Trump's reelection campaign is also gradually attempting to recover momentum.

A key for Mr Trump will be either getting the economy fired up or at least persuading voters that he has done everything to protect them.

He insisted again that he is optimistic.

"I feel much different today than I did two or three days ago," he said. "I'll not rest until that prosperity has been fully restored."

However, he is walking a political minefield on whether to encourage a quick end to social distancing, in hopes of businesses coming back, or to remain cautious in case the virus rebounds.

Mr Trump said he has told the governor of Georgia, who is aggressively reopening the state economy, that he "strongly" disagrees with those measures, a rare rebuke from the president to a Republican ally.

At the same time, he played down the risks of a second wave of coronavirus hitting the US this autumn, right when the election season is peaking.

"We may not even have (coronavirus) coming back," he said.

This was bluntly contradicted by Anthony Fauci, the country's top infectious diseases specialist and a key White House advisor.

"There will be coronavirus in the fall," he stated.