NEW YORK >> Thousands of nurses went on strike Monday at two of New York City’s major hospitals after contract negotiations stalled over staffing and salaries nearly three years into the coronavirus pandemic.

The privately owned, nonprofit hospitals were postponing nonemergency surgeries, diverting ambulances to other medical centers, pulling in temporary staffers, and assigning administrators with nursing backgrounds to work in wards in order to cope with the walkout.

As many as 3,500 nurses at Montefiore Medical Center in the Bronx and about 3,600 at Mount Sinai Hospital in Manhattan were off the job. Talks were resuming Monday afternoon at Montefiore, but there was no immediate word on when bargaining might resume at Mount Sinai.

Hundreds of nurses picketed, some singing the chorus from Twisted Sister’s 1984 hit “We’re Not Gonna Take It,” outside Mount Sinai. It was one of many New York hospitals deluged with COVID-19 patients as the virus made the city an epicenter of deaths in spring 2020.

“We were heroes only two years ago,” said Warren Urquhart, a nurse in transplant and oncology units. “We was on the front lines of the city when everything came to a stop. And now we need to come to a stop so they can understand how much we mean to this hospital and to the patients.”

The nurses union, the New York State Nurses Association, said members had to strike because chronic understaffing leaves them caring for too many patients.

Jed Basubas said he generally attends to eight to 10 patients at a time, twice the ideal number in the units where he works. Nurse practitioner Juliet Escalon said she sometimes skips bathroom breaks to attend to patients. So does Ashleigh Woodside, who said her 12-hour operating-room shifts often stretch to 14 hours because short staffing forces her and others to work overtime.

“We love our job. We want to take care of our patients. But we just want to do it safely and in a humane way, where we feel appreciated,” Woodside said.

The hospitals said they had offered the same raises — totaling 19% over three years — that the union had accepted at several other facilities where contract talks reached tentative agreements in recent days.

Montefiore said it had agreed to add 170 more nurses. Mount Sinai’s administration said the union’s focus on nurse-to-patient ratios “ignores the progress we have made to attract and hire more new nurses, despite a global shortage of healthcare workers that is impacting hospitals across the country.”

The hospitals said Monday that they had prepared for the strike and were working to minimize the disruption. Mount Sinai called the union’s behavior “reckless,” while Montefiore said the strike was sparking “fear and uncertainty across our community.”

“In my opinion, this action was totally unnecessary,” Montefiore President Dr. Philip Ozuah told staffers in a memo Monday afternoon. Ozuah maintained that the two sides had been close to agreement on “a very generous offer.”

Some patients, meanwhile, were left in limbo.

Darcy Gervasio took medical leave from her job at a suburban college library, made child care and transportation arrangements, got tests and otherwise prepared for a gastrointestinal surgery that was scheduled Monday but now is postponed indefinitely, she said. While the procedure is considered elective, Gervasio said it’s essential to controlling her Crohn’s disease.

“As a patient, of course, I am annoyed and inconvenienced,” she wrote in an email. But Gervasio, a union member herself, said she blames the hospital management, not the nurses.

“I am very disappointed in the administration for letting the nursing staffing crisis get out of hand in the first place — especially in the wake of the tremendous strain on nurses during the COVID pandemic,” Gervasio wrote. She questioned why Mount Sinai couldn’t strike a deal with the union when several other local hospitals did in the last two weeks.

Gov. Kathy Hochul urged the union and the hospitals late Sunday to take their dispute to binding arbitration. Montefiore’s administration had said it was willing to let an arbitrator settle the contract; the union did not immediately accept the proposal. In a statement, it said Hochul, a Democrat, “should listen to the frontline COVID nurse heroes and respect our federally-protected labor and collective bargaining rights.”

A lineup of other city and state Democratic politicians, including Attorney General Letitia James, joined a midday union rally Monday.

Both hospitals had prepared for the walkout by transferring patients, including intensive-care newborns at Mount Sinai. State Health Department representatives were at the two medical centers Monday to monitor staffing levels, the agency said.

Montefiore and Mount Sinai are the last of a group of hospitals with nursing contracts that expired simultaneously. The union initially warned that it would strike at all of them at the same time, but the other hospitals reached agreements as the deadline approached. All include raises of 7%, 6%, and 5% over the next three years.