The United States issued its strongest public rebuke toward Israel on Thursday since the start of the Gaza war, conditioning support for Israel’s offensive on concrete Israeli steps to address the safety of aid workers and Palestinian civilians.

U.S. President Joe Biden, a staunch supporter of the offensive until now, called for an immediate ceasefire in a call with Prime Minister Benjamin Netanyahu following an Israeli attack on a World Central Kitchen charity convoy this week that killed seven food aid workers.

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.”

Biden “made clear that U.S. policy with respect to Gaza will be determined by our assessment of Israel’s immediate action on these steps,” the White House said in a statement.

The statement reflected a sharp change in Biden’s tone and, for what appears to be the first time, a set of strings attached to continued U.S. support. Biden staunchly supported Israel, even when other governments sought to put more pressure on Israel.

His comments marked the first time the U.S. has suggested it would condition its continued support.

By suggesting a shift in U.S. policy toward Gaza was possible if Israel did not address the humanitarian situation in the Palestinian enclave, Biden channeled his own frustration along with mounting pressure from his left-leaning political base in the Democratic Party to stop the killings and alleviate hunger among innocent civilians.

During a briefing with reporters after the call, White House spokesperson John Kirby declined to elaborate on what specific changes the U.S. would make on its policy toward Israel and Gaza.

He said Washington hopes to see an announcement of Israeli steps in the “coming hours and days.”

On Monday, Israel launched an attack that killed seven workers with the World Central Kitchen group, founded by celebrity chef Jose Andres. Andres told Reuters in an interview on Wednesday that the Israeli attack had targeted his aid workers “systematically, car by car.”

Israel said on Thursday it would adjust tactics in the Gaza war after describing the attack as the result of a misidentification, and that inquiry findings would be made public soon.

The White House has described Biden as outraged and heartbroken by the attack but, prior to Thursday’s call, the president had made no fundamental change in Washington’s steadfast support for Israel in its conflict against Palestinian Hamas militants.

During the call, Biden “underscored that an immediate ceasefire is essential to stabilize and improve the humanitarian situation and protect innocent civilians,” the White House said. Biden urged Netanyahu to empower his negotiators to conclude a deal to bring home hostages captured by Hamas in its deadly Oct. 7 attack, it added.

In Brussels, U.S. Secretary of State Antony Blinken said Israel “must meet this moment” by surging humanitarian assistance and ensuring the security of those who provide aid.

“If we don’t see the changes that we need to see, there’ll be changes in our policy,” Blinken told reporters.