WASHINGTON (Reuters) -The Federal Aviation Administration said it will unveil details of its plan on Thursday to cut 10% of flights at 40 high-traffic U.S. airports to address safety concerns due to the shortage of air traffic controllers during a record-setting U.S. government shutdown.

The busiest airports will be affected, including the three largest in the New York City area and others in Chicago, Dallas, Los Angeles, Houston, Phoenix, Tampa, Miami, Denver, Boston, Atlanta, San Diego, Washington and Detroit, according to a list seen by Reuters.

Transportation Secretary Sean Duffy announced the plan on Wednesday, citing internal safety data on the performance of air traffic controllers who have been working without pay for more than a month during the government shutdown.

FAA Administrator Bryan Bedford told CEOs of major airlines in a call late Wednesday the cuts would begin with 4% on Friday, rise to 5% on Saturday, 6% on Sunday, and 10% next week.

However, the FAA is likely to revise the plan and only require 4% cuts through Sunday, 6% on Monday, and reach 10% only on Wednesday, airline officials told Reuters.

The requirements are expected to take effect early on Friday, the officials added. The FAA is expected to issue an order outlining requirements shortly.

The FAA did not immediately comment on the expected changes to the proposed cuts.

Major airlines including American Airlines, United Airlines, Southwest Airlines and Delta Air Lines are waiving change fees and letting passengers rebook or cancel flights over the weekend for free because of the uncertainty.

The plan will exempt international flights and the cuts will apply to flights between 6 a.m. and 10 p.m. The FAA is also imposing drastic restrictions on space launches and general aviation.

Airlines were left scrambling to make significant reductions in flights in just 36 hours, and passengers flooded airline customer service hotlines with their concerns about air travel in the coming days.

Carriers are also pressing for details including how the cuts are to be spread across the day, while some wanted to begin canceling Friday flights on Thursday in order to notify and reaccommodate passengers.

The government shutdown, the longest in U.S. history, has forced 13,000 air traffic controllers and 50,000 Transportation Security Administration agents to work without pay.

Tens of thousands of flights have been delayed since the shutdown began because of widespread air traffic control shortages. Airlines say at least 3.2 million travelers have already been affected by air traffic control shortages.

Some airline CEOs also pressed Bedford for more details on the undisclosed safety data that led the FAA to take such drastic action.

U.S. Representative Rick Larsen, the top Democrat on the committee that oversees the FAA, called on the agency to explain its "dramatic and unprecedented step."

"The FAA must immediately share any safety risk assessment and related data that this decision is predicated on with Congress," he added.

National Transportation Safety Board Chair Jennifer Homendy praised the move by Duffy to cut flights. "What he's doing is taking proactive action to make sure nothing happens," she said.