"This is about where’s the pressure and how do we alleviate the pressure," Transportation Secretary Sean Duffy said.

Hundreds of flights at the busiest airports in the U.S. are being scratched this weekend. Airlines are moving forward with reducing air service due to the lingering government shutdown.

Transportation Secretary Sean Duffy announced 40 major airports would face a 10% cut in flights. See the full list of impacted airports.

Follow live updates as flight reductions begin today at 40 high-traffic airports in what officials have described as a "proactive" effort to alleviate pressure points.

A small percentage of domestic flights were canceled on Friday, but Transportation Secretary Sean Duffy said the reductions, which were already going to increase next week, could sharply rise by the holidays if the shutdown continued.

As many as 1,800 flights could be affected on Friday alone, according to Cirium, an aviation-data provider. The final figure will depend on which airports are targeted. The FAA estimates that it handles an average of more than 44,000 flights a day.

As the longest government shutdown in U.S. history drags on, airlines have been ordered to begin reducing operations on a phased-in basis.

Transportation Secretary Sean Duffy said Friday that flight reductions could go as high as 20 percent if the government shutdown drags on, as the Federal Aviation Administration (FAA) begins reducing flights by 10 percent due to air traffic controller staffing shortages.