President Trump announced a plan to close the United States to people trying to come to the country to live and work. He justified the drastic move as a necessary step to protect American workers from foreign competition once the nation’s economy begins to recover from the shutdown caused by the coronavirus outbreak.

It appears many cases will still continue to be accepted and processed, but they will be held short of final approval to travel into the United States for as long as the suspension lasts. There is no way to foresee how long this will be at this time.

Under the kind of executive order the president described, the Trump administration would no longer approve any applications from foreigners to live and work in the United States for an undetermined period of time — effectively shutting down the legal immigration system in the same way the president has long advocated closing the borders to illegal immigration.

Workers who have for years received visas to perform specialized jobs in the United States would also be denied permission to arrive, though workers in some industries deemed critical could be exempted from the ban, people familiar with the president’s decision said.

The president’s announcement caught some senior Department of Homeland Security officials off guard, and the agency did not respond to questions and requests to explain Trump’s plan late Monday.

The United States already has placed broad restrictions on travel from Europe, China and other pandemic hot spots, while implementing strict controls at the country’s land borders. International air travel has plummeted.

Halting immigration to the United States could affect hundreds of thousands of visa holders and other would-be green card recipients who are planning and preparing to come to the United States at any given time. Most of them are the family members of Americans.

It remains unclear what exceptions Trump could include in such a sweeping immigration order, or if would-be immigrants could reach the United States by demonstrating they are free of the virus. The White House officials said they thought the order would not be in place long-term.

On March 18, the State Department canceled most routine immigrant and nonimmigrant visa appointments at its offices overseas, effectively shutting down almost all new kinds of travel into the United States. The State Department also stopped all processing for refugee resettlement.

Later that week, however, authorities resumed processing H-2A visas for seasonal guest workers. The country's agricultural laborers have been officially declared “essential workers,” including hundreds of thousands of people who enter the country under that temporary visa.

Trump already has cited the health emergency to enact the kind of enforcement measures at the U.S. border with Mexico he has long extolled, moves that have essentially closed the border to asylum seekers and waved off anti-trafficking protections for underage migrants. During the past few weeks of the coronavirus crisis, U.S. border authorities have expelled 10,000 border crossers in an average of just a little more than an hour and a half each, which has effectively emptied out U.S. Border Patrol holding facilities of detainees.

U.S. border authorities say the measures are in place to help federal agents, health-care workers and the public by preventing potentially infected migrants from crossing into the United States, while minimizing the population of detainees in U.S. immigration jails.