NEWARK/WASHINGTON (Reuters) -The union representing more than 13,000 air traffic controllers on Monday urged workers to remain on the job during the ongoing partial government shutdown that requires them to work without pay.

U.S. Transportation Secretary Sean Duffy and the president of the National Air Traffic Controllers Association are holding a press conference on Monday at Newark Liberty International Airport to discuss the impacts of the shutdown. The airport near New York City is a major United Airlines hub.

The union told workers that "participating in a job action could result in removal from federal service" and is illegal.

"It is more important than ever that we rise to the occasion and continue delivering the consistent, high-level of public service we provide every day," the union told members. "We cannot stress enough that it is essential to avoid any actions that could reflect poorly on you, our union, or our professions."

About 50,000 Transportation Security Administration employees who staff airport security checkpoints are also required to keep working, but are not getting paid.

In 2019, during a 35-day shutdown, the number of absences by controllers and TSA officers rose as workers missed paychecks, extending checkpoint wait times at some airports. Authorities were forced to slow air traffic in New York, which put pressure on lawmakers to quickly end the standoff.

NATCA in 2019 said many controllers had reached "the breaking point." Then-House Speaker Nancy Pelosi said at the time the shutdown was "pushing our airspace to the breaking point."

Airline trade group Airlines for America, which represents United, Delta Air Lines American Airlines and Southwest Airlines ( LUV ), warned that during a funding lapse "the system may need to slow down, reducing efficiency" and impacting travelers.

Congress in July approved $12.5 billion to begin a massive overhaul over five years. A persistent shortage of controllers has delayed flights and many are working mandatory overtime and six-day weeks. The FAA is about 3,500 air traffic controllers short of targeted staffing levels.