WASHINGTON (Reuters) -U.S. Transportation Secretary Sean Duffy warned Friday he could force airlines to cut up to 20% of flights if the government shutdown did not end, as U.S. airlines on Friday scrambled to make unprecedented government-imposed reductions.

The Federal Aviation Administration instructed airlines to cut 4% of flights on Friday at 40 major airports because of the government shutdown. The cuts will rise to 10% by November 14.

Separately, air traffic controller absences on Friday forced the FAA to delay hundreds of flights at 10 airports including Atlanta, San Francisco, Houston, Phoenix, Washington, D.C., and Newark. By 7:30 p.m. ET (1130 GMT), there were more than 5,300 flight delays, according to FlightAware, a flight-tracking website.

At Reagan Washington National, delays were averaging four hours, while 17% of flights were cancelled and nearly 40% delayed.

During the record 38-day government shutdown, 13,000 air traffic controllers and 50,000 security screeners have been forced to work without pay, leading to increased absenteeism. Many air traffic controllers were notified on Thursday that they would receive no pay for a second pay period next week.

The Trump administration has sought to ramp up pressure on Democrats in Congress to agree to a Republican plan to fund the federal government, which would allow it to reopen.

Raising the specter of dramatic air-travel disruptions is one such effort. Democrats contend Republicans are to blame for the shutdown because they refuse to negotiate over extending health insurance subsidies.

Duffy told reporters it was possible he could require 20% cuts in air traffic if things get worse and more controllers do not show up for work. "I assess the data," Duffy said. "We're going to make decisions based on what we see in the airspace."

The cuts, which began at 6 a.m. ET (1100 GMT), include about 700 flights from the four largest carriers - American Airlines, Delta Air Lines, Southwest Airlines and United Airlines - and are set to rise to 6% on Tuesday and then 10% by November 14 if the shutdown does not end.

New York Governor Kathy Hochul posted a photo of an airport flight board filled with canceled flights. "The GOP shutdown has grounded America - just in time for the holidays!" she wrote.

Earlier this week, FAA Administrator Bryan Bedford said 20-40% of controllers were not showing up for work on any given day.

MORE CUTS WOULD BE 'PROBLEMATIC', AMERICAN AIRLINES SAYS

American Airlines CEO Robert Isom said on Friday he did not expect significant disruption for customers from government-ordered flight reductions initially, but warned that the impact would increase.

"This level of cancellation is going to grow over time and that's something that is going to be problematic," Isom told CNBC.

American told Reuters its 220 flight cancellations on Friday affected 12,000 passengers and the airline re-routed a majority of them within a few hours.

Fewer flights will be cut over the weekend as scheduled flight volumes decline.

United Airlines said half of affected customers were able to be rebooked within four hours of their original departure time. The airline canceled 184 flights on Friday and expected to cut 168 on Saturday and 158 on Sunday.

Duffy initially announced on Wednesday that flights would be reduced 10% on Friday. But the agency decided to phase in the cuts starting at 4% to make the move less disruptive, he said on Friday.

Duffy said safety data was behind the move, including incidents of planes not maintaining separation and ground incursions.

The FAA is restricting space launches, and authorities said they could cut up to 10% of private-plane flights at high-traffic airports. International flights are not affected by the move.