Air traffic controllers lost contact with aircraft heading to and from Newark Liberty International Airport last week, their union said, detailing an equipment failure that led to massive flight delays and raised more concerns about aging U.S. aviation infrastructure and staffing shortages.

The controllers who guide flights in and out of the New Jersey airport on April 28 "temporarily lost radar and communications with the aircraft under their control, unable to see, hear, or talk to them," the National Air Traffic Controllers Association, their union, said in a statement.

Staffing shortages followed the incident, which was so severe that some of the controllers involved "have taken time off to recover from the stress of multiple recent outages," the Federal Aviation Administration said on Monday.

There were more than 1,500 delays in the New Jersey airport last week, according to flight-tracker site FlightAware, as disruptions piled up because of shortages of air traffic controllers.

"Our antiquated air traffic control system is affecting our workforce," the FAA said. "We are working to ensure the current telecommunications equipment is more reliable in the New York area by establishing a more resilient and redundant configuration with the local exchange carriers."

The FAA and union did not say how long the outage lasted, but Bloomberg reported, citing people familiar with the matter, that it was nearly 90 seconds.

United Airlines said Friday that it will cut 35 flights a day from its New York City area hub at Newark because of the delays, in hopes of putting more slack into the system and ease disruptions.

In a note to customers, CEO Scott Kirby said Friday that last week's "technology issues were compounded as over 20% of the FAA controllers for EWR walked off the job."

"This particular air traffic control facility has been chronically understaffed for years and without these controllers, it's now clear — and the FAA tells us — that Newark airport cannot handle the number of planes that are scheduled to operate there in the weeks and months ahead," Kirby said in his note.

The union denied that the controllers walked off the job and explained that workers took time off under the Federal Employees Compensation Act, which "covers all federal employees that are physically injured or experience a traumatic event on the job."

The U.S. has faced a shortage of air traffic controllers for years. The Trump administration recently rolled out new incentives to hire and retain controllers, who are required to retire at age 56.

The FAA last year moved controllers who are responsible for aircraft arriving and departing from Newark from a facility on Long Island in New York to a different facility in Philadelphia, in hopes of reducing overloaded controllers who were also handling traffic for New York City's major airports.

The airspace is some of the most congested in the world.

"The Port Authority has invested billions to modernize Newark Liberty, but those improvements depend on a fully staffed and modern federal air traffic system," the Port Authority of New York and New Jersey, which oversees the major airports in the New York City area, said in a statement Monday. "We continue to urge the FAA to address ongoing staffing shortages and accelerate long-overdue technology upgrades that continue to cause delays in the nation's busiest air corridor."

U.S. Transportation Secretary Sean Duffy last week visited the Philadelphia facility and said he will unveil plans for an "brand new air traffic control system" this week.

"The system that we're using is not effective to control the traffic that we have today," he told reporters last week.

Despite the aging technology, Duffy stressed that the system is safe because the FAA will slow, if not ground, airplanes altogether if air traffic controllers have capacity constraints.