More than 7,000 nurses walked off the job at two major city hospitals Monday after contract talks broke down overnight, threatening potential chaos at the crucial Big Apple facilities.

The massive staffing blow at Mount Sinai Hospital’s main campus in Manhattan and three locations of Montefiore Medical Center in the Bronx was set to force the hosps to divert ambulances and some patients, including women about to give birth, to other facilities, officials said.

The strike-battered hospitals also were rejigging their staff to cover nursing shifts and canceling non-emergency surgeries.

Montefiore typically has the busiest emergency room in the state, according to its Web site. 

Oren Barzilay, president of the FDNY’s EMS union, told The Post on Monday that emergency workers were preparing for the worst and would reroute their vehicles from affected hospitals if the facilities “can’t handle a certain medical emergency.”

As officials struggled to maintain health-care services at the hospitals, the striking nurses began picketing Monday. An estimated 450 nurses gathered outside Mount Sinai on the Upper East Side, singing Twisted Sister’s classic hit “We’re Not Gonna Take it” and urging passing drivers to honk their horns in solidarity.

Some of the workers on the scene noted that their profession carried the country through its COVID crisis — and is continuing to do so. As The Post reported Sunday, coronavirus deaths  in New York rose again last month to their highest level in nearly a year. 

“We were heroes only two years ago,” Mount Sinai nurse Warren Urquhart said. “We [were] 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.”A protesting nurse practitioner, Juliet Escalon, added,

“We were so-called heroes — and now, this is how you treat the heroes?”

At issue at the private hospitals are staffing levels and pay, which the New York State Nurses Association union says are woefully lacking.

While other city hospitals repped by the nurses union have reached tentative settlements, averting strikes there, talks involving the two hold-out hospitals dissolved when the union walked away from the table around 1 a.m. Monday, Mount Sinai said. 

“We made some progress at negotiations at both tables last night but not enough,’’ a union source told The Post on Monday. “At both Mt. Sinai and Montefiore, the main sticking point is understaffing that harms patient care —  neither hospital has been willing to accept full accountability for enforcement of safe staffing levels in our contracts yet.”

But Mount Sinai scoffed in a statement, “Not only do they continue to demand additional wage increases, they also continue to make demands on staffing, though they responded to our offer to hire an additional 50 nurse positions by stating they would prefer those dollars to instead be reallocated to wages.”

The hospital added, “We are in complete agreement about the importance of attracting and retaining more nurses. … Over the last three years, we’ve hired more than 4,000 new nurses with 503 more nurses working today than in 2019 — far exceeding our 2019 hiring commitment to NYSNA.”

As for the battle over salaries, the hospital said in a release Monday, “NYSNA leadership … refused to accept the exact same 19.1 percent increased wage offer agreed to by eight other hospitals, including two other Mount Sinai Health System campuses.”

Montefiore described Monday’s walkout as “a sad day for New York City.

“Despite Montefiore’s offer of a 19.1% compounded wage increase — the same offer agreed to at the wealthiest of our peer institutions — and a commitment to create over 170 new nursing positions … NYSNA’s leadership has decided to walk away from the bedsides of their patients,” the medical center said.

Some nurses said the pay hike touted by the hospitals breaks down to three years’ worth of raises: about 7%, 6% and 5% annually. 

But more importantly, the hospitals refuse to hire enough staff, leaving patients at risk and them exhausted, the nurses said.

“I’m supposed to have six patients max. I have eight or nine regularly, and if I have to cover someone, that 16 or 18. That’s not good!’’ Mount Sinai nurse Darla Joiner, 40, told The Post outside the hospital Monday.

“Safe staffing is the most important issue to all of us here today,’’ she said. “It seems like after COVID, nurses weren’t being hired. And the new ones that were hired had no clinical experience at all.

“New nurses [are] working 24-hour shifts, and it’s becoming the norm. That’s the most dangerous thing — one mistake can mean someone’s life.”

A longtime nurse who only gave her first name, Patricia, 56, said, “All the problems go straight back to staff retention.

“[Mount Sinai] can’t keep experienced nurses like me to train the new people because we’re the lowest-paid hospital. The new nurses are training the new nurses, and the patient care is deplorable.”

The affected locations in the Bronx are Montefiore Moses on East 210th Street, Montefiore Weiler on Eastchester Road and Montefiore Hutch on Waters Place.

Gov. Kathy Hochul called on all of the parties to agree to binding arbitration to end the mess.

Mayor Eric Adams had warned as the strike loomed, “Hospitals in certain areas may experience impacts to operations, including possible delayed or limited service.“

We encourage all New Yorkers to call 911 only for emergencies, and be prepared to seek an alternate facility in case their preferred hospital is impacted.”

The union remained non-commital. Both Mount Sinai and Montefiore said they would agree to arbitration.

Meanwhile, nurses at Mount Sinai Morningside and Mount Sinai West, both in Manhattan, came to an agreement with management Sunday afternoon.

Additional reporting by Joe Marino and AP