The Federal Aviation Administration will announce the 40 “high-volume markets” where it is reducing flights by 10% before the cuts go into effect Friday, said agency administrator Bryan Bedford. The move is intended to keep the air space safe during the shutdown, the agency said.

Experts predict hundreds if not thousands of flights could be canceled. The cuts could represent as many as 1,800 flights and upwards of 268,000 seats combined, according to an estimate by aviation analytics firm Cirium.

“I’m not aware in my 35-year history in the aviation market where we’ve had a situation where we’re taking these kinds of measures,” Bedford said Wednesday. “We’re in new territory in terms of government shutdowns.”

Air traffic controllers have been working unpaid since the shutdown began Oct. 1. Most work mandatory overtime six days a week, leaving little time for side jobs to help cover bills and other expenses unless they call out.

Mounting staffing pressures are forcing the agency to act, Bedford said Wednesday at a news conference.