WASHINGTON, Oct 30 (Reuters) -

Flights at Orlando, Dallas/Fort Worth and Washington, D.C.

airports were delayed on Thursday due to a shortage of air

traffic controllers during the U.S. government shutdown, the

Federal Aviation Administration said.

Delays averaged more than two hours at Orlando, 90

minutes at National Airport in Washington and 21 minutes at

Dallas, the agency said.

FlightAware, a flight tracking website, said nearly

6,000 flights had been delayed and 1,100 canceled nationwide on

Thursday. About 20% of flights were affected at United Airlines

, American Airlines ( AAL ), Southwest Airlines ( LUV )

and Delta Air Lines ( DAL ), FlightAware said.

Since the government shutdown began on October 1, more

than 13,000 air traffic controllers have been required to work

without pay. Some are taking second jobs to make ends meet, and

the number calling in sick has increased.

The FAA had planned to close Orlando for landings on

Thursday night due to insufficient staff but reversed that when

it secured enough controllers.

Transportation Secretary Sean Duffy said this week that 44%

of delays on Sunday and 24% on Monday were caused by air traffic

controller absences, compared to 5% on average before the

shutdown.

The FAA is about 3,500 air traffic controllers short of

targeted staffing levels and many had been working mandatory

overtime and six-day weeks even before the shutdown.