US Transportation Secretary Sean Duffy warned on Tuesday that if the federal government shutdown continues another week it could lead to “mass chaos” and could force him to close some of the national airspace to air traffic, a drastic move that could upend American aviation.

“If you bring us to a week from today, Democrats, you will see mass chaos, you will see mass flight delays. You’ll see mass cancellations, and you may see us close certain parts of the airspace, because we just cannot manage it,” Duffy said at a press conference Tuesday.

US aviation has already faced tens of thousands of flight delays over the last month. The comments represented the Trump administration’s most dire warnings of impending impacts from rising air traffic controller absences.

As the shutdown entered its 35th day, the Federal Aviation Administration delayed a new round of flights on Tuesday. Airlines said this week 3.2 million passengers have been hit by delays or cancelled flights due to a spike in air traffic controller absences.

FAA Administrator Bryan Bedford told Fox Business’ Varney and Co on Tuesday that at the agency’s largest 30 airports “anywhere from 20 to 40 per cent of our controllers aren’t coming to work”,

Democrats blame Republicans for not negotiating with them to reopen the government.