Airlines rushed to provide travelers updates this week after the Federal Aviation Administration said it would reduce flights across 40 airports as the longest government shutdown in history continues to drag on.

Many major airlines said they would waive cancellation fees for even their most basic tickets, which often come with penalties for changes.

Transportation Secretary Sean Duffy previously said he would reduce flight capacity by roughly 10%, affecting 3,500 to 4,000 flights daily.

On Thursday, the FAA formalized a list of affected airports and clarified the reductions would begin at 4% and slowly ramp up to 10% by Nov. 14. The reductions began Friday morning with more than 700 flight cancellations.

These are the airports that are expected to be impacted, including some of the country’s largest airports and major international hubs in Atlanta, Chicago, Dallas, Los Angeles and New York City.

(The airport in Las Vegas was renamed the Harry Reid International Airport in 2021.)

On Wednesday, Duffy said the reduction was a “proactive” measure because of the delays and cancellations already occurring due to the shutdown. Air traffic controllers, who are considered essential employees required to work during a shutdown, have missed paychecks, and the FAA has said the closure has also raised concerns about already thin staffing among controllers.

Duffy said he expects more cancellations as a result of the reduction, which has no set end time.

“We thought 10% was the right number based on the pressure we were seeing,” Duffy added.

Earlier this week, Duffy told CNBC’s “Squawk Box” that he could “shut the whole airspace down” if the shutdown drags on.

FAA Administrator Bryan Bedford said Wednesday that additional measures may be implemented after the reduction, which he said he has never seen before in his time in the industry. The officials said they were planning to meet with airlines to discuss which flights would be cut.

In a Wednesday memo to United Airlines employees, CEO Scott Kirby said the carrier will not be reducing long-haul international flying and hub-to-hub flying, instead reducing regional and domestic flights that do not fly between hubs.

The airline also offered all customers refunds even if their flights are not impacted. Kirby said that included “non-refundable tickets and those customers with basic economy tickets.”

On Thursday afternoon, the airline preemptively said it was going to cancel 4% of its flights from Friday through Sunday.

In a statement, Delta Air Lines said it expects to operate the “vast majority” of its flights as scheduled and will offer changes, cancellations or refunds for customers’ flights during the impacted period. Delta also said that would include basic economy fares, without penalty.

The airline added on Thursday afternoon that it will cancel flights a day in advance to allow customers enough time.

Frontier Airlines CEO Barry Biffle said he highly recommends travelers flying Friday or in the next 10 days book a backup ticket on another carrier as the flight reductions begin to avoid getting stranded due to cancellations.

“I’m sorry this is happening. Hopefully the shutdown is over soon,” Biffle wrote on LinkedIn. “Just giving everyone practical travel advice.”

American Airlines said it expects that the “vast majority of customers’ travel will proceed as planned,” adding that the carrier will reach out to travelers proactively as schedule changes occur.

The airline also said that it will offer immediate rebooking options for all impacted travelers and that customers whose flights are canceled for any reason will be able to change their flight or request a refund without penalty. As of Thursday morning, the airline was still awaiting clarifying information from the FAA about which of its flights will be impacted.

Southwest Airlines also released a statement saying that the majority of its flights will not be impacted and that its international flights should operate as usual. The airline said it will “proactively communicate well in advance and will offer flexibility in travel plans.”

The Association of Flight Attendants, representing 55,000 flight attendants at 20 airlines, released a statement Wednesday urging Congress to end the shutdown so air traffic controllers and Transportation Security Administration workers can get paid.

“The false narrative that this shutdown is a choice of either paying federal workers or protecting affordable healthcare is outrageous when both crises were manufactured by the exact people who can fix it,” the statement read.

Consumers have been buying travel insurance at an elevated rate amid the government shutdown, but travel and insurance experts warn that such policies don’t offer blanket protection for shutdown-related travel snafus, and a lot depends on the fine print.

For example, a policyholder generally can’t get insurance benefits if they choose to cancel their travel plans to avoid any headaches. Cancel-for-any-reason coverage is an exception, though it also comes with its own caveats.

Whether or not a policyholder gets compensated may come down to the rationale an airline provides for a delayed or canceled flight.

Many insurers only pay benefits if a delay or cancellation is attributable to a “common carrier” disruption like a mechanical failure, travel experts said.

“Airlines typically won’t cite causes other than operational terms like ‘mechanical issues’ or general delays, cancellations, or lost belongings, even during a government shutdown,” Lauren McCormick, a spokesperson for Squaremouth, an online platform for comparing travel insurance policies, wrote in a recent blog post. “So, these are generally still covered under most comprehensive travel insurance plans.”

— CNBC’s Phil LeBeau contributed to this report.