The Trump administration will announce on Friday that the US is closing its southern border with Mexico in an effort to slow the spread of the coronavirus.

"We're looking at both our northern and our southern border," said Chad Wolf, the acting head of the Department of Homeland Security.

He said that the new measures would restrict "non-essential" travel, and that an announcement would be made on Friday. Members of the White House's coronavirus task force, possibly joined again by Donald Trump, are slated to brief reporters at 11:45 a.m.

The US has already closed its northern border with Canada. Mr Trump said this week that he has been in discussions with Mexico to do the same.

"We're working with both our Canadian partners as well as our Mexican partners to put some restrictions on the travel across that border, really looking at limiting non-essential travel across that border," Mr Wolf told reporters outside the White House on Friday morning. "We want to make sure that cargo continues, trade continues, health care workers continue to be able to traverse that border."

Meantime, a day after the president said his administration is considering using military aircraft to bring home some Americans now overseas but want to return to US soil, Mr Wolf urged them to consider commercial airlines.

"There's a lot of commercial flights out there," Mr Wolf said.

"There are certain countries where they are locking down their borders and US citizens will find themselves in those countries," he said. "So the State Department is certainly facilitating that, just like they did in China, but I would encourage all Americans that are back overseas [that] they see the travel restriction, you don't have to get on the plane right away."

"We don't need a mad rush into our airports," the acting DHS chief said, repeating the administration's policy of making sure anyone returning who has the virus does not return into the general population and potentially spread it to others. "We're going to screen you. We're going to take you."