American and United airlines agreed yesterday to cut or reschedule 37 flights to Chicago's O'Hare International Airport, and other carriers promised to adjust their schedules, also, as part of an effort led by the Federal Aviation Administration to curb congestion at the nation's most delay-prone hub.

The deal essentially freezes the number of O'Hare flights for other airlines while forcing the two largest airlines that have added the most flights to cut them back. The cuts and flight adjustments will result in a 20 percent decrease in delays at O'Hare and a 5 percent decrease in delays nationwide, the FAA said. The airlines did not say which routes will be affected.

Under the agreement that takes effect Nov. 1 and expires April 30, 2005, airlines must apply for government approval if they want to add new flights at O'Hare or swap time slots with another carrier. Airlines that violate the agreement face stiff fines, federal officials said.

"Everybody saw the benefit of coming to an agreement voluntarily rather than having to have something imposed on them by us," said Transportation Secretary Norman Y. Mineta, who announced the deal at O'Hare yesterday with Federal Aviation Administrator Marion C. Blakey. The two officials warned carriers earlier this month that if they did not reach an agreement voluntarily, the Transportation Department would tell carriers how often they could fly into the airport for the first time since Congress deregulated the industry in 1978.

The deal announced yesterday came after two weeks of meetings the FAA held in Washington separately with 15 airlines that operate at O'Hare. Under supervision of the Justice Department, which enforces antitrust laws, the FAA asked each carrier to offer adjustments without knowing what other carriers had offered.

Critics of government involvement in private industry said the deal would result in higher airfares because they would have limited choices at the airport. "We would have preferred that the marketplace . . . solve these congestion problems, rather than a group of bureaucrats in Washington," said David S. Stempler, president of the Air Travelers Association.

Delays at O'Hare often disrupt flights across the country because so many passengers connect through the hub on either American or United airlines, which together operate 88 percent of flights at the airport. As air travel has returned to levels not seen since the 2001 terrorist attacks, both carriers and new low-fare airlines have added flights that sometimes cause more traffic than the airport can handle, which results in hours-long delays and the common sight of passengers sleeping on cots laid out in the terminal.

Between 2000 and 2003, UAL Corp.'s United Airlines has increased flights by 41 percent at O'Hare, and AMR Corp.'s American Airlines has increased flights by 10 percent. Yesterday, American and United said they would cut or reschedule 17 and 20 incoming flights, respectively, that arrive during the airport's busiest periods of the day. United also agreed to change the arrival time of one flight. The carriers said that they expected some passengers would be inconvenienced by the changes this fall but that overall flights would be less prone to delay.

United, which is operating under Chapter 11 bankruptcy protection, said it did not expect the cuts would result in further financial harm. "Delays cost us money, so anything we can do to improve delays at O'Hare not only helps our customers but helps United as well," said Jean Medina, a company spokeswoman.

Other airlines, such as Flyi Inc.'s Dulles-based Independence Air did not agree to cut flights but offered to move some arrival times by a half-hour to less-congested periods of the day. The agreement doesn't prevent carriers from changing the kind of aircraft they use at O'Hare, said Rick DeLisi, Independence Air spokesman. Theoretically, he said, the airline could replace its 50-seat jets that it flies to O'Hare with larger Airbus planes that it expects to receive this fall.

"This is an excellent solution," DeLisi said. "We hope to operate in an environment less challenging than O'Hare has been in the past."

The airline industry and the FAA said they recognized the agreement represented just a short-term solution to the congestion problem at the critical hub. The bigger need, several carriers said, is for another runway at O'Hare. The union representing air traffic controllers said the FAA also needs to add more controllers in Chicago.

American said in a prepared statement that "long-term solutions need to focus on increasing capacity at O'Hare rather than restricting schedules. Reducing schedules adversely affects smaller cities that are dependent on access to the nation's air transportation system through O'Hare."

Yesterday, it was unclear whether the same clogged situation will return in April, when the new agreement expires.

Mineta said it's possible the FAA will need to renegotiate flight schedules at O'Hare next year.

"We'll have to monitor that," Mineta said. "It's sort of hard to prejudge where we'll be on April 30, given the nature of the airline industry and their financial condition."

Federal Aviation Administrator Marion C. Blakey, left, and Transportation Secretary Norman Y. Mineta discuss O'Hare changes.Delays at O'Hare International Airport became problematic enough for the government to step in.