All three airlines said they would cap fares for flights between Europe and the United States. Delta said on Wednesday night that it would waive change fees for all customers traveling to, from or through Europe and the United Kingdom through May.

For two days, United’s chief executive, Oscar Munoz, has been in Washington, meeting with senior Trump administration officials and members of Congress to discuss the policies they are considering and to explain to them the effect the virus is having on United’s business, Mr. Munoz and United’s president, Scott Kirby, said in a letter to employees on Thursday, which was viewed by The New York Times.

The men also said that Mr. Kirby had been speaking with executives at large companies, including banks, to understand the effect the outbreak is having on the broader economy and that United’s operational and finance teams were developing contingency plans to prepare for the virus’s spread.

Confusion reigned after Mr. Trump’s announcement, with frantic travelers struggling to understand the consequences of the ban, which does not apply to American citizens and permanent residents and their close relatives, and airlines racing to adjust operations on little notice. The airlines would not say publicly whether the White House had notified them of the ban before it was announced.

“This action will hit U.S. airlines, their employees, travelers and the shipping public extremely hard,” said Nicholas Calio, president of Airlines for America, an industry group. “However, we respect the need to take this unprecedented action and appreciate the administration’s commitment to facilitate travel and trade.”

But others in the industry were far more critical. Sara Nelson, the president of the Association of Flight Attendants, the union representing flight attendants at 20 airlines, including United, said in an interview that the airline industry was “pretty blindsided” by Mr. Trump’s announcement.

“It created total pandemonium,” Ms. Nelson said. “Airline employees had no idea what this meant and for some it was completely disrupting their lives. They weren’t even sure if they were going to have a job in the next 24 hours or get home.”