WASHINGTON (AP) — President Joe Biden issued a stark warning to Israeli Prime Minister Benjamin Netanyahu on Thursday that future U.S. support for Israel's Gaza war depends on the swift implementation of new steps to protect civilians and aid workers.

Biden and Netanyahu 's roughly 30-minute call just days after Israeli airstrikes killed seven food aid workers in Gaza added a new layer of complication to the leaders' increasingly strained relationship. Biden's message marks a sharp change in his administration's steadfast support for Israel's war efforts, with the U.S. leader for the first time threatening to rethink his backing if Israel doesn't change its tactics and allow much more humanitarian aid into Gaza.

The White House would not specify what could change about U.S. policy, but it could include altering military sales to Israel and America's diplomatic backup on the world stage. Administration officials said they expected the Israelis to make announcements on next steps within hours or days and that the U.S. would then assess whether the Israeli moves go far enough.

Biden "made clear the need for Israel to announce and implement a series of specific, concrete, and measurable steps to address civilian harm, humanitarian suffering, and the safety of aid workers," the White House said in a statement following the leaders' call. "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 also told Netanyahu that an "immediate cease-fire is essential" and urged Israel to reach such an accord "without delay," according to the White House, which described the conversation as "direct" and "honest."

There was no immediate reaction to the call from the Israeli government.

The leaders' conversation comes as the World Central Kitchen, founded by restauranteur José Andrés to provide immediate food relief to disaster-stricken areas, called for an independent investigation into the Israeli strikes that killed the group's staff members, including an American citizen.

The White House has said the U.S. has no plans to conduct its own investigation even as it called on Israel to do more to prevent the harming of innocent civilians and aid workers as it carries out its operations in Gaza.

Separately, Secretary of State Antony Blinken told reporters in Brussels that U.S. support would be curtailed if Israel doesn't make significant adjustments to how it's carrying out the war. "If we don't see the changes that we need to see, there will be changes in our policy," he said.

White House national security spokesman John Kirby echoed the call for "tangible" and "concrete" changes to be taken by the Israelis beyond reiterating long stated calls for allowing additional aid to get into Gaza.

"If there's no changes to their policy in their approaches, then there's going to have to be changes to ours," Kirby said. "There are things that need to be done. There are too many civilians being killed."

The demands for Israel to bring the conflict to a swift close were increasing across the political spectrum, with former President Donald Trump, the Republicans' presumptive nominee to face Biden this fall, saying Thursday that Israel was "absolutely losing the PR war" and calling for a resolution to the bloodshed.

"Get it over with and let's get back to peace and stop killing people. And that's a very simple statement," Trump told conservative radio host Hugh Hewitt. "They have to get it done. Get it over with and get it over with fast because we have to -- you have to get back to normalcy and peace."

The two leaders also discussed Iranian threats against Israel, Kirby said. Earlier this week, Iranian leaders vowed to hit back after an airstrike widely blamed on Israel destroyed Iran's Consulate in Syria, killing 12 people, including two elite Iranian generals. Iran's President Ebrahim Raisi said Wednesday the attack "will not remain without answer."

Biden also renewed his concerns about Netanyahu's plan to carry out an operation in the southern city of Rafah, where about 1.5 million displaced Palestinians are sheltering, as Israel looks to eliminate Hamas following the militant group's deadly Oct. 7 attack. Vice President Kamala Harris, Blinken and national security adviser Jake Sullivan also joined the call.

Still, the Biden administration has proceeded apace with arms transfers and deliveries to Israel, many of which were approved years ago but had only been partially or not at all fulfilled. Just this week, on Monday, the Democratic administration's "Daily List" of munitions transfers included the sale to Israel of more than 1,000 500-pound (225-kilograms) bombs and more than 1,000 1,000-pound (450-kilogram) bombs.

Officials said those transfers had been approved before the publication of the list on Monday — the day Israeli airstrikes hit a World Central Kitchen aid convoy in Gaza, killing seven of the group's employees — and that they fell below the threshold for new congressional notification. Also, they noted that the bombs are not for delivery to Israel until 2025.

Israel has acknowledged responsibility for the strikes but said the convoy was not targeted and the workers' deaths were not intentional. The country continues to investigate the circumstances surrounding the killings.

Andrés harshly criticized the Israeli military for the strike, and his organization has paused its work in Gaza.

"The Israeli government needs to stop this indiscriminate killing. It needs to stop restricting humanitarian aid, stop killing civilians and aid workers, and stop using food as a weapon," he wrote on X. "No more innocent lives lost."

The war in Gaza began when Hamas-led militants stormed into southern Israel, killing some 1,200 people, mostly civilians, and taking around 250 people hostage.

The Israeli military campaign in Gaza, experts say, is among the deadliest and most destructive in recent history. Within two months, researchers say, the offensive already has wreaked more destruction than the razing of Syria's Aleppo between 2012 and 2016, Ukraine's Mariupol or, proportionally, the Allied bombing of Germany in World War II. It has killed more civilians than the U.S.-led coalition did in its three-year campaign against the Islamic State group.

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AP writers Matthew Lee in Brussels, Jill Colvin in New York and Colleen Long and Chris Megerian contributed.