A Delta Air Lines Boeing 767-300 landing in Amsterdam. Nicolas Economou | NurPhoto | Getty Images

Delta Air Lines and American Airlines announced deep flight cuts and other cost-saving measures Tuesday as the coronavirus poses the biggest threat to travel demand since the Sept. 11, 2001, terrorist attacks. Delta said it is reducing its international flights by as much as 25% and domestic capacity between 10% and 15%, among some of the deepest cuts announced in the U.S. so far. With so much uncertainty, the airline suspended its financial outlook for this year, which other U.S. carriers have also done.

For its part, American said it will shave 10% off its peak summer international flying, one of the clearest signs yet that airline executives expect the coronavirus's impact on the business to last longer than expected. That includes a 55% cut to its trans-Pacific flying capacity as travel demand throughout Asia plummets. The virus originated in Wuhan, China, in December and quickly spread throughout that country and to nearby South Korea, Singapore and Japan. It will also cut April domestic capacity by 7.5%. American, which had already suspended service to mainland China for several weeks, extended that to October. The carriers' cuts follow similar measures taken at United and JetBlue last week. President Donald Trump late Monday said his administration is working with industries hard hit by the coronavirus, including airlines, hotels and cruise lines, but he did not provide specifics on what aid could look like. Bookings have dropped sharply in recent days as more cases of the coronavirus have been reported in the U.S. and booking trends will likely worsen, Delta CEO Ed Bastian said at the J.P. Morgan Industrials Conference, which was changed to a webcast last week from a live event amid increasing corporate precautions to prevent the spread of the virus. Airline executives at the conference sought to calm nervous investors, playing up their stronger balance sheets compared with previous crises when they were saddled with debt and faced more domestic competitors. They also said a sharp drop in fuel prices could help soften the blow. But they also detailed the impact of the coronavirus. As the virus spread outside of Asia, Delta registered a "25% to 30% decline in net bookings and are prepared for it to get worse," Bastian said. "We expect demand erosion to continue in the near term and have built a plan that prioritizes free cash flow generation and preserves liquidity."