President Donald Trump's decision to suspend immigration to the United States for 60 days to stop those applying for permanent residency from taking American jobs has drawn criticism from immigration advocates who brand it a "distraction" amid the coronavirus pandemic.

"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!" the president tweeted Monday night.

Trump said he signed the executive order Wednesday. "This would ensure that unemployed Americans of all backgrounds will be first in line for jobs as our economy opens," Trump said at Wednesday night's coronavirus news briefing.

The president had initially wanted to suspend all immigration to the US to stop the spread of COVID-19 and "protect American jobs", as 22 million have filed for unemployment. But on Tuesday he said his executive order would affect only some family members of US citizens seeking green cards (permanent residency IDs) and foreign workers who want to move to America.

"It would be wrong and unjust for Americans laid off by the virus to be replaced with new immigrant labor flown in from abroad," Trump said at the briefing.

The executive order won't bar immigrants already living in America seeking green cards, or the 85,000 workers a year given H-1B visas nor seasonal farm workers. It also will not stop people using temporary visas for work or travel.

"The Trump administration is again seeking to distract Americans from their own failures to secure testing, provide basic protections for all workers, and create a healthcare system that works for us all," BittaMostofi, commissioner of the New York Mayor's Office of Immigrant Affairs, told China Daily.

"Among the 1 million essential workers in New York City working on the front lines of the COVID-19 pandemic — delivery workers, EMS staff, drivers, healthcare personnel, and more — half are immigrants."

Former US secretary of state John Kerry told CNN that the immigration order was "a sideshow, an effort to divert attention".

The president's initial announcement caught some senior officials at the Department of Homeland Security off guard Tuesday, The Washington Post reported.

Trump has often used immigration as an issue to appeal to his political base. He pledged to build a wall along the Mexican border and enforced a travel ban from a number of Muslim-majority countries.

Amid COVID-19, he restricted travel from Europe and China. He also closed the border with Canada and Mexico.

The US now has the most COVID-19 cases of any country with some 838,000 infected and at least 46,000 dead as of Wednesday, according to data from Johns Hopkins University.

Legal experts noted that if Trump had barred all immigration, he likely would have faced a court challenge.

"If the executive order (had) suspended all immigration to the United States, it would surely be challenged as unconstitutional," Stephen Yale-Loehr, professor of immigration law at Cornell Law School and an expert on immigration and asylum law in the US, told China Daily.

"Presidents have wide authority on immigration, but no president has suspended all immigration before. There would definitely be challenges to the executive order. … It is one thing to require extra vetting to make sure foreign nationals entering the United States are healthy. It is another thing to ban all foreign nationals on the fallacious theory that they all have COVID-19."

Any order that affected current green-card holders "from returning to the United States would surely have been challenged as violating the due process rights of those individuals", Yale-Loehr said.

Ian Kysel, a visiting assistant clinical professor of law at Cornell Law School and director of the International Migrants Bill of Rights Initiative, also believes Trump could not have banned all immigration to the US.

Said Kysel: "Such an action would have (had) devastating consequences for families, universities, businesses and communities around the country. Even more than doubling down on an apparently xenophobic effort to dismantle the US immigration system, a blanket ban unmoored from public health imperatives violates basic international human rights law obligations."

The State Department canceled immigrant and nonimmigrant visa appointments on March 18, halting the issuance of visas. Processing for refugee resettlement has also been paused.

Over the past three years, the number of visas issued to people who want to immigrate to the US fell to 462,422 in the 2019 fiscal year, down from 617,752 in 2016.

In 2019, the State Department issued about 462,422 immigration visas, according to The Washington Post. Citizenship and Immigration Services, part of the Department of Homeland Security, processed and approved nearly 580,000 green cards.

At least 6 million US healthcare workers were born abroad, including 29 percent of all doctors, 38 percent of home health aides and 23 percent of retail store pharmacists, according to the Migration Policy Institute.