Hundreds of flights are slashed, and more could be cut if the shutdown doesn’t end. What to know if you’re about to fly

(CNN) — Travel plans for millions of Americans could come to a screeching halt this month as more flights get canceled during the longest government shutdown in US history.

More than 1,000 Friday flights were slashed on the first day of the Trump administration’s mandatory cuts to reduce US air traffic, a majority due to the cuts mandated because of the shutdown. Officials said the cancellations are needed to maintain safety, as some air traffic controllers and federal security screeners – who aren’t getting paid during the shutdown – are not showing up to work.

But the reduction in flights has not eased the cascading delays for travelers across the country due to air traffic controller staffing shortages, as more than 4,500 flights were delayed Friday.

The cancellations and delays are mostly due to a combination of air traffic controller staffing issues and an Federal Aviation Administration-mandated 4% cut to flights because of the shutdown.

Friday’s cuts represent a 4% reduction in flights at 40 of the largest airports nationwide. If the shutdown drags on, airlines would have to gradually cancel more flights in the coming days – potentially leading to 10% of flights axed next Friday.

Here’s what to know as bewildered travelers get hit by the shutdown:

Airlines must cancel flights to 40 “high-volume traffic” airports, the FAA said. Each airline can decide which specific flights to cut, as long as they meet the mandatory number of cancellations.

Northeast: Boston Logan International, Newark Liberty International, New York John F. Kennedy International, New York LaGuardia, Philadelphia International and Teterboro in New Jersey.

Midwest: Chicago Midway International, Chicago O’Hare International, Cincinnati/Northern Kentucky International, Detroit Metropolitan Wayne County, Indianapolis International, Louisville Muhammad Ali International and Minneapolis-St. Paul International.

South: Charlotte Douglas International, Dallas-Fort Worth International, Dallas Love Field, Fort Lauderdale-Hollywood International, George Bush Houston Intercontinental, Hartsfield-Jackson Atlanta International, Houston Hobby, Memphis International, Orlando International, Miami International and Tampa International.