NEW YORK (AP) — Thousands of nurses in three hospital systems in New York City went on strike Monday after negotiations through the weekend failed to yield breakthroughs in their contract disputes.

Nurses were to start walking off the job at 6 a.m. at The Mount Sinai Hospital and two of its satellite campuses. The other affected hospitals are NewYork-Presbyterian and Montefiore Medical Center in the Bronx.

About 15,000 nurses are involved in the strike, according to New York State Nurses Association.

The strike, which comes during a severe flu season, could potentially force the hospitals to transfer patients, cancel procedures or divert ambulances. It could also put a strain on city hospitals not involved in the contract dispute, as patients avoid the medical centers hit by the strike.

The hospitals involved have been hiring temporary nurses to try and fill the labor gap during the walkout, and said in a statement during negotiations that they would “do whatever is necessary to minimize disruptions.” Montefiore posted a message assuring patients that appointments would be kept.

The work stoppage is occurring at multiple hospitals simultaneously, but each medical center is negotiating with the union independently. Several other hospitals across the city and in its suburbs reached deals in recent days to avert a possible strike.

The nurses’ demands vary by hospital, but the major issues include staffing levels and workplace safety. The union says hospitals have given nurses unmanageable workloads.

Nurses also want better security measures in the workplace, citing incidents like a an incident last week, when a man with a sharp object barricaded himself in a Brooklyn hospital room and was then killed by police.

The union also wants limitations on hospitals’ use of artificial intelligence.

The nonprofit hospitals involved in the negotiations say they’ve been working to improve staffing levels, but say the union’s demands overall are too costly.

Nurses voted to authorize the strike last month.

Both New York Gov. Kathy Hochul and Mayor Zohran Mamdani had expressed concern about the possibility of the strike. As the strike deadline neared, Mamdani urged both sides to keep negotiating and reach a deal that “both honors our nurses and keeps our hospitals open.”

“Our nurses kept this city alive through its hardest moments. Their value is not negotiable,” Mamdani said.

The last major nursing strike in the city was only three years ago, in 2023. That work stoppage, at Mount Sinai and Montefiore, was short, lasting three days. It resulted in a deal raising pay 19% over three years at those hospitals.