U.S. Transportation Secretary Ray LaHood says thousands of jobs are at risk because Congress failed to approve funding for the Federal Aviation Administration.

LaHood on Monday said there is “no excuse” for Congress not passing the spending bill. He said the funding is crucial to the economy and tens of thousands of middle-class families.

The FAA says congressional authorization for several programs expired on Saturday, forcing nearly 4,000 employees to be furloughed without pay.

LaHood and FAA administrator Randy Babbitt told reporters in a conference call Monday that Congress needs to reauthorize the funding. They stressed that the funding issue will not impact airline safety, adding that air traffic controllers are on duty and the U.S. air system continues to be the safest in the world.

The Department of Transportation says FAA employees and thousands of construction workers, engineers and planners for airport projects across the country were not able to report for work. The projects included construction on new air traffic control towers and a project to demolish an old tower.

Federal airline ticket taxes have been suspended. But some airlines have already raised prices, hoping to capitalize on the drop in overall ticket prices.

Lawmakers failed to pass the funding for the FAA because of a dispute between Democrats and Republicans on subsidies to rural communities.