WASHINGTON (Reuters) -Transportation Secretary Sean Duffy said Monday the Trump administration would shutter the U.S. aviation system if he thought an ongoing government shutdown was making it too risky to travel.

"If we thought that it was unsafe... we'll shut the whole airspace down. We won't let people travel. We're not there at this point. It's just significant delays," Duffy told CNBC, adding "absolutely there's more risk" during the ongoing shutdown.

United CEO Scott Kirby said last week the ongoing shutdown was impacting flight bookings and airlines are concerned about the looming start of the holiday travel season.

The shutdown has forced 13,000 air traffic controllers and 50,000 Transportation Security Administration officers to work without pay and snarled tens of thousands of flights.

Duffy also said he has no plans to fire air traffic controllers who are calling in sick, saying they "are trying to put food on their families' table. I am asking all of them to come to work."

A growing number of air traffic controller absences during the 34-day-old government shutdown have led to dramatic delays at U.S. airports. Absences by Transportation Security Administration security officers have led to very long lines at San Diego and Houston airports in recent days. Houston Bush had more than three-hour-long security lines on Sunday.

On Friday, the Federal Aviation Administration said nearly half of the 30 busiest U.S. airports faced shortages of air traffic controllers, leading to more than 6,200 flights being delayed and 500 canceled, according to FlightAware, a flight tracking website.

In New York on Friday 80% of air traffic controllers were absent, the agency said. Duffy said 65% of delays Friday were caused by controller absences.

On Saturday there were 4,600 delays and 173 canceled flights, and on Sunday 5,800 delays and 244 canceled flights. On Monday as of 9 a.m. ET, there have been 750 flights delayed and 54 canceled.

AIRLINES URGE CONGRESS TO END SHUTDOWN

Delta Air Lines ( DAL ), United, Southwest Airlines ( LUV ), American Airlines ( AAL ) and the National Air Traffic Controllers Association have all called on Congress to quickly pass a stop-gap funding bill to let the government reopen amid talks on disputes over healthcare policy.

The government shutdown began on October 1 and continued as a federal funding bill has stalled in Congress.

Airlines have repeatedly urged an end to the shutdown, citing aviation safety risks.

The shutdown has exacerbated existing staffing shortages, threatening to cause widespread disruptions similar to those that helped end a 35-day government shutdown in 2019. 

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.