The N.A.A.C.P., the oldest and largest civil rights group in the nation, called on Thursday for President Biden to “draw the red line” and halt weapons shipments to Israel over the mounting civilian death toll in its war in Gaza.

In a rare foray into foreign policy, the influential organization added to the mounting pressure from Black leaders on Mr. Biden to stop aiding Israel’s war in Gaza. Its warning comes as Mr. Biden tries to shore up softening support among Black Americans, a constituency that was crucial in catapulting him to the White House in 2020 and that he will need to win his re-election bid in November.

In its statement, the N.A.A.C.P. called on Mr. Biden to “draw the red line and indefinitely end the shipment of weapons and artillery” to Israel and any states that supply weapons to terrorist organizations, including Hamas.

“The Middle East conflict will only be resolved when the U.S. government and international community take action, including limiting access to weapons used against civilians,” the statement said.

The N.A.A.C.P.’s announcement came the same day that an Israeli airstrike in central Gaza killed dozens of people at a United Nations school complex that had become a shelter for thousands of displaced Palestinians.

It tightens the political bind Mr. Biden finds himself in as he pushes for a cease-fire to end the war while continuing to provide support for a longtime U.S. ally. He has recently withheld some offensive weapons from Israel and has threatened to hold back more, but has also made clear that he will continue to supply defense systems and arms that aid in Israel’s “ability to respond to attacks” like one Iran launched in April.