Oct. 7 (UPI) — As the U.S. government shutdown drags into its second week, the Federal Aviation Administration was forced to delay flights into and out of three U.S. airports Monday due to air traffic controller staffing issues, including one airport with no controllers.

Newark Liberty International Airport, Denver International Airport and Hollywood Burbank Airport all experienced delays after a rising number of controllers called out sick.

Air traffic controllers are not being paid during the government shutdown, but are considered essential and are required to work.

“So, we’re tracking sick calls, sick leave and have we had a slight tick up in sick calls? Yes, and then you’ll see delays that come from that,” Transportation Secretary Sean Duffy told reporters Monday at Newark.

“If we have additional sick calls, we will reduce the flow consistent with a rate that’s safe for the American people,” Duffy said, adding that he did not want to see flights canceled.

Hollywood Burbank Airport in the Los Angeles area remained open Monday despite having no controllers show up for work during a six-hour stretch.

“Operations are continuing at Hollywood Burbank Airport,” the airport wrote in a post Monday. “Please check with your airline — before arriving at the airport — for updates on possible delays or cancellations.”

Air traffic into and out of Burbank was handled by San Diego TRACON on a delayed schedule, a source told NBC News.

Flights into Newark, near New York City, experienced delays of up to an hour Monday due to low air traffic controller staffing, while flights into Denver were delayed by an average of 39 minutes.

The U.S. government has been shut-down since Sept. 30, and will remain closed for a seventh day. On Monday, the U.S. Senate failed to pass a stopgap bill that would have funded federal agencies for the next six weeks.