A shortage of air traffic controllers are delaying flights, the Federal Aviation Administration warned on Wednesday, as concerns grow about the effect of the government shutdown on U.S. aviation.

There were half-hour delays at Ronald Reagan Washington National Airport late Wednesday afternoon due to staffing, the FAA said.

About 10,000 flights were delayed on Monday and Tuesday, though disruptions dropped on Wednesday to just over 3,200. Delays could be caused by weather or other reasons aside from staffing issues. But a shortfall of already-thin air traffic control staffing this week had prompted the FAA to slow or halt arrivals in Burbank, California, and Nashville, Tennessee, among others.

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