The government shutdown is now disrupting travel plans for thousands.

Air traffic controllers have been working without pay since October 1, and an increasing number of them have been calling in sick as the shutdown extends into its second month.

These staffing shortages prompted the Federal Aviation Administration to mandate a 10% reduction in flights at 40 of the country's busiest airports, forcing airlines to cancel flights.

That started with a 4% reduction on Friday. It will increase to 10% by November 14.

Some 750 flights have so far been canceled as of 6 a.m. ET on Friday, according to data from Cirium, an aviation analytics firm. Only domestic trips are directly affected by the FAA mandate, with major airline hubs bearing the brunt of the impact.

Transportation Secretary Sean Duffy said at a news event in Washington, DC on Friday afternoon that if the shutdown continues for much longer, more unpaid air traffic controllers will stop showing up for work.

He said that could mean a mandated reduction of air traffic as high as 20%.

Transportation Secretary Sean Duffy said at a news event in Washington, DC on Friday afternoon that if the shutdown continues for much longer, more unpaid air traffic controllers will stop showing up for work.

He said that could mean a mandated reduction of air traffic as high as 20%.

The Federal Aviation Administration has already grounded 10% of flights. Those cuts began on Friday and will continue to ramp up over the next week unless the shutdown is resolved.