President Joe Biden told Israel‘s Prime Minister Benjamin Netanyahu that an immediate ceasefire was needed in Gaza “to stabilize and improve the humanitarian situation and protect innocent civilians,” the White House said today.

The president urged Netanyahu to “empower his negotiators to conclude a deal without delay to bring the hostages home.” Negotiations for a ceasefire have hinged on Hamas releasing hostages.

Biden spoke by phone to Netanyahu for about 30 minutes, following an Israeli strike that killed seven aid workers for chef José Andrés’ World Central Kitchen.

Biden also told Netanyahu that Israel had to “announce and implement a series of specific, concrete, and measurable steps to address civilian harm, humanitarian suffering, and the safety of aid workers,” according to the White House. The president also signaled a potential policy shift, as he “made clear that U.S. policy with respect to Gaza will be determined by our assessment of Israel’s immediate action on these steps.”

Biden told Netanyahu that the strike on the aid workers “unacceptable,” having previously said that he was “outraged” over what had happened.

Netanyahu has apologized and promised an investigation, but Andrés, in an interview with Reuters, called for an independent investigation. “What I know is that we were targeted deliberately, nonstop until everybody was dead in this convoy,” he said.

According to the White House, Biden and Netanyahu “also discussed public Iranian threats against Israel and the Israeli people. President Biden made clear that the United States strongly supports Israel in the face of those threats.”