By David Shepardson

WASHINGTON (Reuters) - United Airlines said on Tuesday that an acute shortage of air traffic controllers was causing "significant disruption" for travelers at its busy Newark, New Jersey, hub outside of New York City.

The airline said the Federal Aviation Administration has been forced to reduce traffic flows to Newark hub because of low staffing on 12 of the first 25 days of November, disrupting more than 343,000 United travelers by delays, cancellations, long taxi times and longer flight times related to air traffic control delays for Newark.

United said that on Nov. 15 alone, air traffic control staffing issues resulted in canceled flights that disrupted 1,880 customers; gate and other delays disrupted an additional 24,558.

The FAA said in "the Newark airspace, the FAA is addressing a decades-long issue of staffing and has been transparent with airlines and travelers about our plan."

United's comments come as a record-setting Thanksgiving holiday air-travel period is beginning.

"This is why it continues to be imperative for FAA to rebuild staffing levels so travelers can depend on safe, efficient air travel," United said.

Over the last two years, a series of near-miss incidents has raised concerns about U.S. aviation safety and the strain on understaffed air traffic control operations. The FAA said last month it was opening an audit into runway incursion risks at the 45 busiest U.S. airports after a series of near-miss incidents.

The FAA required 17 air traffic controllers to move from New York Terminal Radar Approach Control (TRACON), known as N90, to Philadelphia in late July. New York TRACON is one of the busiest U.S. facilities.

FAA Administrator Mike Whitaker told reporters last week the move allowed the agency "to relieve the stress in New York and improve controller staffing levels by recruiting and training controllers in Philadelphia." Whitaker added that delays were down with the transfer of the airspace.

In recent years, the FAA has been forced to routinely assign controllers six-day work weeks and slow air traffic in the New York area.

The FAA is about 3,000 controllers behind staffing targets and the agency said last year it had 10,700 certified controllers, about the same as a year earlier.

The FAA in June extended cuts to minimum flight requirements at congested New York City-area airports through October 2025, saying the number of controllers handling traffic in New York was insufficient for normal traffic levels.

(Reporting by David Shepardson in Washington; Editing by Matthew Lewis)