The United States will reduce the number of flights by 10% at its 40 largest airports. The reason is a threat to air traffic control safety under a record 36-day government shutdown, Reuters reports.

US Transportation Secretary Sean Duffy announced new flight regulations. The reductions will affect the main airports in New York, Washington, Chicago, Atlanta, Los Angeles, and Dallas.

This would reduce as many as 1,800 flights and over 268,000 airline seats, according to aviation analytics firm Cirium. International flights, however, will remain unchanged for now.

The shutdown, the longest in US history, has forced 13,000 air traffic controllers and 50,000 Transportation Security Administration agents to work without pay.

Tens of thousands of flights have been delayed since the shutdown began because of widespread air traffic control shortages. Airlines say at least 3.2 million travelers have already been impacted by air traffic control shortages.

The US Secretary of Transportation said the cuts could be reversed if Democrats agreed to reopen the government.

The US Federal Aviation Administration also plans to exempt international flights from the cuts.

Airlines are calling for an end to the shutdown, noting the risks to passenger safety.

More than 2,100 flights were delayed on Wednesday, and some air traffic controllers were absent from work, increasing the workload on the remaining staff.