Air traffic controllers briefly lost communication with planes at Newark Liberty International Airport last week, according to the workers’ union, a revelation that came as travel disruptions there extended into a second week.

Galen Munroe, a spokesman for the union, the National Air Traffic Controllers Association, said that on April 28, controllers in a Philadelphia air traffic control center who are responsible for separating and sequencing aircraft in and out of Newark Airport “temporarily lost radar and communications with the aircraft under their control,” and were “unable to see, hear, or talk to them.”

He did not say how long the disruption lasted, but Bloomberg reported it was 90 seconds.

The communication breakdown led to hundreds of delays and cancellations and three dozen flight diversions that day, according to Aidan O’Donnell, the general manager of New Jersey airports at the Port Authority of New York and New Jersey. He added that for two hours that afternoon, no flights departed from or landed at Newark.

As a result of the loss of communication, Mr. Munroe said, controllers took absences under a law that allows federal workers who are physically injured or experience a traumatic event on the job to leave work. They did not “‘walk off the job’ as it has been reported by the media,” Mr. Munroe said in a statement.

The Federal Aviation Administration acknowledged in a statement on Monday that “our antiquated air traffic control system is affecting our work force.” It said it was working to ensure that telecommunications equipment is more reliable in the New York area.

“Frequent equipment and telecommunications outages can be stressful for controllers,” the F.A.A. said.

Some of the controllers in Philadelphia who help to coordinate arrivals and departures at Newark “have taken time off to recover from the stress of multiple recent outages,” the agency added. “While we cannot quickly replace them due to this highly specialized profession, we continue to train controllers who will eventually be assigned to this busy airspace.”

The disclosure comes as one of Newark’s three runways has been closed for construction and as air traffic control centers nationwide have experienced staffing shortages. United said last week that it was forced to cut 35 round-trip flights per day from its Newark schedule.

Low clouds on Monday prompted the F.A.A. to pause departures of planes heading to Newark, leading to delays averaging four hours and exacerbating the travel chaos at one of the nation’s busiest airports. More than 300 flights into and out of Newark had been delayed and more than 150 had been canceled by Monday afternoon, according to the tracking site FlightAware.

At the main United terminal at Newark on Monday, travelers whose flights had been canceled expressed frustration with being directed to online customer service agents.

Phyllis Dotzen Rod said she was hoping to fly home to Myrtle Beach, S.C., after visiting her son in Manhattan, but her flight was canceled after she arrived at the airport. Her son was leaving for Asia and she was not sure what to do, she said.

“I’m stressed right now,” Ms. Dotzen Rod said as she waited in line at a help desk at Terminal C that closed just as she got to the front of the line. “Now I don’t know where else to go.”

Adding to her frustration, she said, she had been given a voucher for a meal and a hotel, but could not figure out how to get it to appear on her phone.

Judith Davis, whose flight home to Columbus, Ohio, was canceled because of the bad weather, said she had waited for 45 minutes on the phone for a customer service agent. She was among the travelers desperately searching for alternative flights at Terminal C on Monday.

“I’m very upset; I need to get back today,” Ms. Davis said, expressing frustration with the lack of help in the terminal. “You’re kind of left to your own to try to figure it out.”

Senator Chuck Schumer of New York on Monday called for the Office of the Inspector General to investigate the problems at Newark, saying a “real forensic look” into safety issues and outdated technology was needed.

“To say that there is just minor turbulence at Newark Airport and the F.A.A. that would be the understatement of the year,” Mr. Schumer, the minority leader, said at a news conference. “We’re here because the F.A.A. is really a mess.”

He said the problems at Newark could be a “harbinger, if issues like these aren’t fixed.” He blamed mismanagement at the F.A.A. and cuts imposed by the Trump administration for the staffing issues, and warned that the nation’s other airports could experience similar problems if they are not addressed.

The Port Authority of New York and New Jersey, which operates Newark Airport as well as Kennedy International and LaGuardia Airports in New York, said in a statement on Monday that staffing shortages at air traffic control centers were to blame.

“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 said. “We continue to urge the F.A.A. to address ongoing staffing shortages and accelerate long-overdue technology upgrades that continue to cause delays in the nation’s busiest air corridor.”

In a statement on Friday, Scott Kirby, the chief executive of United Airlines, Newark’s largest carrier, attributed recent flight cancellations to equipment failures and said that 20 percent of air traffic controllers at the airport had “walked off the job.”

As a result, he added, there were “dozens of diverted flights, hundreds of delayed and canceled flights and worst of all, thousands of customers with disrupted travel plans.”

About 68 percent of the more than 3,300 scheduled departures at Newark this week were sold by United, according to Cirium, an aviation data firm.

Paul Rinaldi, a former president of the National Air Traffic Controllers Association who is now a senior vice president of operations and safety at Airlines for America, a trade organization, said the systems controllers rely on have not been working “at an optimal level.”

“The issue is a lack of confidence by the controllers in the systems because of the interruptions they have had over the last eight months or so,” he said.

Last week, Sean Duffy, the U.S. transportation secretary, announced a series of incentives that he said would “supercharge the air traffic controller work force,” including $5,000 payments to new hires and academy graduates who successfully complete the initial qualification training.

On Monday, Gov. Philip D. Murphy of New Jersey wrote Mr. Duffy in support of that plan. “Decades of underinvestment in the maintenance of critical air traffic control infrastructure, delays in upgrading to modern 21st-century air traffic control technology, and inadequate air traffic control staffing have resulted in a frail system nationwide,” Mr. Murphy wrote.

It was not clear when the delays at Newark Airport would clear up, and bad weather was likely to contribute to the headaches for travelers at Newark as well as at the other metro-area airports.

A Delta spokesman said that the airline had canceled three regional round-trip flights at Newark because of air traffic control constraints. Passengers on those flights were automatically rebooked on flights at LaGuardia and Kennedy Airports.

But those airports were also affected by the weather. Inbound and outgoing flights at LaGuardia were experiencing delays of about an hour because of low clouds.

The clouds and rain may limit flights in and out of the region until midweek. Rain may increase in intensity on Monday, with some thunderstorms also possible. The chance of showers will linger into Wednesday.

Judson Jones and Niraj Chokshi contributed reporting.