Thousands of nurses in New York City went on strike Monday after negotiations through the weekend did not produce breakthroughs in disputes with three major hospital systems over staffing, benefits and other issues, according to the New York State Nurses Association.

On picket lines outside hospitals including NewYork-Presbyterian Hospital in Upper Manhattan, nurses shouted for a contract. Nurses also picketed at hospitals in the Mount Sinai and Montefiore systems, where a 2023 nursing strike had followed pandemic-era frustrations and ended with a deal to boost staffing and pay.

Emergency department nurse Tristan Castillo, speaking Monday outside Mount Sinai West, said the dispute was not just about pay. “They don’t want to give us a fair contract, and they don’t want to give us safe staffing, and now they’re trying to roll back on our benefits,” Castillo said.

NYSNA said about 15,000 nurses are involved in the strike. The walkout affects private, nonprofit hospitals rather than city-run ones, but it could still test how Mayor Zohran Mamdani’s new administration handles a labor standoff involving what the union and visiting politicians describe as essential health work.

Mamdani visited the NewYork-Presbyterian picket line on Monday. “These executives are not having difficulty making ends meet,” he said, adding that nurses were seeking “dignity, respect and the fair pay and treatment that they deserve. They should settle for nothing less.”

Some Democratic city and state politicians also visited striking nurses, and New York Gov. Kathy Hochul sent state health officials to the hospitals. In a statement, Hochul said a negotiation should “recognize[] the essential work nurses do.”

Hospital operators warned that strikes can affect care delivery, but said they were prepared. The strike comes during a severe flu season and could potentially force hospitals to transfer patients, cancel procedures or divert ambulances, though the medical centers said they were committed to meeting patients’ needs. The strike could also add strain to other New York City hospitals if some patients avoid the facilities hit by the walkout.

Nurses’ demands vary by hospital, but staffing levels are a central issue, with the union saying hospitals have given nurses unmanageable workloads. The union also wants better security measures in the workplace, citing an episode last week in Brooklyn in which a man with a sharp object barricaded himself in a hospital room and was then killed by police. The union further wants limitations on hospitals’ use of artificial intelligence.

Hospital representatives said they have improved staffing in recent years and that the union’s overall demands are too costly. Mount Sinai said the union was making “extreme economic demands,” while Montefiore spokesperson Joe Solmonese said the union was pressing “$3.6 billion in reckless demands,” including exorbitant raises.

The hospitals said they remained open and hired temporary nurses to fill gaps. They also put forward pay comparisons for unionized registered nurses: they said the averages are $165,000 a year at Montefiore, $162,000 at Mount Sinai, and $163,000 at NewYork-Presbyterian’s Columbia University Irving Medical Center, with the figures not including benefits. Montefiore said the union’s requests would raise the average to $220,000 in three years, while Mount Sinai said the average there would reach $275,000.

After the nurses gave notice on Jan. 2 of the looming strike, the hospitals vowed to “do whatever is necessary to minimize disruptions” and said they were prepared to deliver care regardless of how long the walkout lasts. Mount Sinai said Monday that it had lined up 1,400 temporary nurses.

New York-Presbyterian accused the union of staging a strike “to create disruption,” while saying it was ready to keep negotiating a “fair and reasonable contract” that reflects respect for nurses and “recognizes the challenging realities of today’s healthcare environment.” The three medical centers are negotiating with the union independently, and other private hospitals in and near New York City reached deals in recent days to avert a possible strike.

The 2023 three-day strike at Mount Sinai and Montefiore resulted in a deal raising pay 19% over three years and included staffing improvements. The union and hospitals now disagree about how much progress has been made since then, including whether hospitals are retreating from staffing guarantees, and they also dispute whether hospitals are trying to reduce health benefits. Mount Sinai said its proposals would cut costs without changing coverage.