President Joe Biden on Thursday made one of his most concrete public calls for Israel to change what it’s doing on the ground in Gaza, going as far as to signal a possible shift in U.S. policy with respect to Gaza if his administration isn’t satisfied with Israel’s next steps.

During a call with Prime Minister Benjamin Netanyahu, Biden ramped up pressure on Israel to take “immediate” action to announce and implement “measurable” steps to address civilian harm and the safety of aid workers.

“He 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 call, which lasted about 30 minutes. “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’s comments to Netanyahu appear to be an escalation of rhetoric amid already high tension between the two leaders following an Israeli airstrike on Monday that killed seven aid workers in Gaza.

White House national security spokesman John Kirby said Thursday during a White House briefing that the Biden administration is looking for “concrete steps” from Israel in the “coming hours and days.”

“If we don’t see changes from their side, there will have to be changes from our side, but I won’t preview what that could look like now,” Kirby said.

The call came after Biden this week issued a statement that he was “outraged and heartbroken” by the deaths of the aid workers from World Central Kitchen.

“They were providing food to hungry civilians in the middle of a war,” Biden said. “They were brave and selfless. Their deaths are a tragedy.”

He went on to say that Israel hasn’t done enough to protect aid workers and civilians in Gaza.

“The United States will continue to do all we can to deliver humanitarian assistance to Palestinian civilians in Gaza, through all available means,” Biden said. “I will continue to press Israel to do more to facilitate that aid.”

Kirby called the airstrike a “tragic incident” that was unintentional.

But the founder of World Central Kitchen, José Andrés, said his aid workers were targeted.

“This was not just a bad luck situation where, ‘Oops, we dropped a bomb in the wrong place,'” Andrés told Reuters.

“They were targeting us in a deconflicting zone, in an area controlled by IDF,” he said. “They, knowing that it was our teams moving on that road … with three cars.”

World Central Kitchen immediately paused operations in the region after the incident, which raises concerns about the availability of aid for civilians moving forward.

Netanyahu called it a “tragic incident of an unintentional targeting of innocent civilians in Gaza that we regret” and pledged an investigation.