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.

The strike was taking place at The Mount Sinai Hospital and two of its satellite campuses, with picket lines forming. 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.