“Shaken” by an Israeli airstrike earlier this week that killed seven food aid workers in Gaza, President Joe Biden on Thursday pressed Israeli Prime Minister Benjamin Netanyahu for major tactical changes in his war with Hamas, a senior White House official said.

The U.S. president has been feeling “growing frustration” in recent weeks as Netanyahu and his government have turned away humanitarian aid and continued to hammer civilian targets in Rafah, where pro-Palestinian activists say more than 1 million refugees are holed up, said White House national security communications adviser John Kirby.

The Biden-Netanyahu rift, amid calls from some of Biden’s Democratic allies to slap new conditions on American military aid to the Jewish state, threatens to upend a decadelong diplomatic partnership.

Biden told Netanyahu on a call that an “immediate cease-fire” is required to improve the humanitarian disaster inside Gaza, Kirby said.

In a summary of the high-stakes call issued moments before Kirby briefed reporters, the White House said 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. 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.”

Arab American activists and some Democratic lawmakers have said in recent weeks that Biden appears to have lost sway over the Israeli leader and called on him to use a threat of withholding weapons like American-made bombs, ammunition, fighter jets and other combat equipment to coerce Netanyahu to use a lighter approach with so many Palestinian civilians in harm’s way.

As humanitarian groups and activists warn of mass starvation inside Gaza, Kirby on Thursday described the situation there as “famine.”

Underscoring the widening split, Biden wants to see changes in Israel’s approach inside Gaza within “hours and days,” Kirby said, describing the president as “shaken” over an Israeli strike that killed seven aid workers.

A direct threat about withholding military aid could be Biden’s next card to play. To that end, Kirby declined to comment when asked about the possible halting of U.S. military aid, but he said Biden wants more border crossings opened for aid.

Delaware Democratic Sen. Chris Coons, a key Biden ally and Foreign Relations Committee member, on Thursday called the strike that killed the World Central Kitchen workers, including a dual U.S.-Canadian national, “hard to explain.”

For the first time, Coons joined other House and Senate Democrats in calling for lawmakers and Biden to place new restrictions on U.S. military aid to Israel.

“I think we’re at that point. And I think we’re at the point where President Biden has said, and I have said and others have said, if Benjamin Netanyahu … were to order the IDF into Rafah at scale — [if] they were to drop thousand-pound bombs and send in a battalion to go after Hamas and make no provision for civilians or for humanitarian aid — that I would vote to condition aid to Israel,” Coons told CNN.

“I’ve never said that before. I’ve never been here before,” added Coons, who is chairman of the Senate Appropriations subcommittee that oversees funding for the State Department and foreign aid. “I’ve been a strong supporter of Israel the whole time I’ve served in Congress.”

Biden has been under pressure at home from Arab American groups and others sympathetic to Palestinian civilians to push Netanyahu toward a less brutal approach to the war in Gaza and to allow more humanitarian aid into the enclave. In Wisconsin, more than 48,000 voters cast protest ballots in Tuesday’s primary by choosing “uninstructed” rather than Biden or other candidates. A group that urged people to vote that way said on a Wednesday call with reporters that Biden’s previous habit of pushing back rhetorically on Israeli leaders has not mattered much when his administration was shipping Netanyahu fighter jets and powerful bombs for use in Gaza, where a Hamas-run health agency says more than 32,000 people have been killed.

Reema Ahmad, director of the Muslim Voter Fund, noted that when Biden beat former President Donald Trump in Wisconsin in 2020, it was by 20,682 votes.

“It means … we have the margin of victory,” Ahmad said. “The pathway to the presidency this November is going to run through Wisconsin. It was the tipping-point state in 2020.”

She and other activists on the call told reporters that while they have not heard directly from Trump’s campaign, they have “open lines of communication” with the state’s Republican Party.

The activists noted that many “uninstructed” organizers want to find a way to cast ballots for Biden on Election Day. But to do so, and drop their push for a protest vote, Biden needs to bring about a cease-fire in Gaza and show some “realistic steps toward peace” in the war-torn region.

“The movement is not about unseating Biden,” Ahmad said, even as the group warned his policies could bring about just that in Wisconsin, long part of Democrats’ Electoral College “blue wall.”