Delta Air Lines will cut passenger-carrying capacity by 40% to deal with a nosedive in travel demand, and it is talking to the White House and Congress about assistance to get through a downturn caused by the new coronavirus.

The 40% cut in capacity is the largest in Delta's history, surpassing reductions that were made after the September 2001 terror attacks.

And the outlook for airlines is still getting worse.

"The speed of the demand fall-off is unlike anything we've seen – and we've seen a lot in our business," CEO Ed Bastian said in a memo to Delta's 90,000 employees Friday.

"We are in discussions with the White House and Congress regarding the support they can provide to help us through this period. I'm optimistic we will receive their support," he said, adding, however, that the airline "can't put our company's future at risk waiting on aid from our government."

Shares of Delta were up 14% in midday trading Friday amid a broader market rally.

Even before Delta's announcement, Treasury Secretary Steven Mnuchin signaled that Washington wants to provide aid to the airline industry.

"We will be coming very quickly back on issues dealing with the airline industry," Mnuchin told CNBC on Friday. "Just as after Sept. 11, we are very committed to make sure that our U.S. airlines have the ability and have the liquidity to get through this."

Delta is the world's largest airline by revenue, and it has consistently been the most profitable U.S. carrier over the past decade.

Just over a week ago, airline CEOs who emerged from a White House meeting with President Donald Trump said they did not ask for help from Washington.

However, the combination of fewer bookings and more cancellations has picked up speed since then, and the airlines' problems were made even more dire by Trump's decision this week to ban most foreigners from flying from Europe to the U.S. for 30 days, beginning at midnight Friday.

Delta, American Airlines and United Airlines announced fresh rounds of flight cutbacks soon after Trump announced the ban.

With bookings plunging because of the coronavirus and travel restrictions imposed by governments, airlines have cut thousands of flights, frozen hiring and increased borrowing to shore up their liquidity.

—CNBC contributed to this report.