Britain's Prime Minister Boris Johnson (R) welcomes US President Donald Trump (L) to the NATO summit at the Grove hotel in Watford, northeast of London on December 4, 2019.

The U.S. will add the U.K. and Ireland to sweeping travel restrictions as the coronavirus spreads, President Donald Trump said Saturday, as he discouraged the American public from unnecessary travel.

Trump earlier this week announced that foreigners who have been in 26 European countries over the last two weeks won't be allowed in to the U.S. for 30 days. Vice President Mike Pence said the rules will take effect midnight Monday.

Airlines have scrambled to slash flights amid the pandemic and unprecedented travel restrictions to stop the spreading virus have crushed demand. Air cargo and ocean freight are exempt from the rules. The new ban to entry on foreigners who have been in the U.K. and Ireland takes effect at midnight Monday.

Speaking at a coronavirus briefing on Saturday, Trump said further travel restrictions, possibly within the United States, from places with high numbers of cases, is possible.

"If you don't have to travel, I wouldn't do it," Trump said, when asked whether people should travel domestically.

Trump said his administration is considering domestic travel restrictions "from certain areas," he said, without elaborating. "We're working with the states and considering other restrictions."

Airlines will likely run flights for several days to allow citizens to return home and then will begin cancellations, according to a person familiar with the matter.

On Friday, a day before Trump added the U.K. and Ireland, Delta Air Lines said it would cut capacity over the next few months by 40%, the most in its history as it scrambles to save cash. It also said it would defer some of its ordered aircraft and park up to 300 of its planes. It is freezing hiring and asking employees to take unpaid leave. Other airlines have also made deep flight cuts and similar staffing changes as they try to preserve jobs.

"The speed of the demand fall-off is unlike anything we've seen – and we've seen a lot in our business," wrote Delta's CEO Ed Bastian in a note to employees.

Trump administration officials have repeatedly said they are working with airlines but it is not yet clear what kind of support the government could offer. Some measures, such as waiving ticket taxes or airport fees could have a limited impact if travel demand falls more steeply or if the Trump administration institutes domestic travel measures, since that money is collected when consumers buy tickets.

"We are in discussions with the White House and Congress regarding the support they can provide to help

us through this period," Bastian said in his note. "I'm optimistic we will receive their support. That said, the form and value is unpredictable, and we can't put our company's future at risk waiting on aid from our government."

United Airlines CEO Oscar Munoz, who is scheduled to hand over the reins to the company's president, Scott Kirby in May, said he met with senior Trump administration officials and lawmakers to "understand what government policies they may be considering and explain to them the impact that the coronavirus has had on our business."