The Biden administration is delaying the sale of at least two arms shipments to Israel amid mounting concern about the country’s plan to expand a military operation in southern Gaza that could dramatically increase the conflict’s death toll, said four people familiar with the matter.

The White House and State Department declined to explain the decision, but it is the first known instance of a delay in U.S. arms transfers since the Oct. 7 Hamas-led attack into Israel that killed more than 1,200 people.