President Joe Biden has threatened to condition support for Israel’s offensive in Gaza on it taking concrete steps to protect aid workers and civilians.

According to Reuters, Biden is seeking for the first time to leverage US aid to influence Israeli military behaviour.

The report said Biden’s warning, relayed in a call with Prime Minister Benjamin Netanyahu on Thursday, came after a deadly Israeli attack on World Central Kitchen aid workers, with Israel saying the attack was a mistake.

The action spurred new calls from Biden’s fellow Democrats to place conditions on US aid to Israel.

Biden, a lifelong supporter of Israel, has resisted pressure to withhold aid or halt the shipment of weapons to the country.

However, the warning marked the first time he has threatened to potentially condition aid, a development that could change the dynamic of the nearly six-month-old war.

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 of the leaders’ phone call. It said the call lasted about 30 minutes.

The president “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.

Washington is Israel’s top weapons supplier and the Biden administration has mostly provided a diplomatic shield for it at the United Nations.

The White House spokesperson, John Kirby, declined to elaborate on any specific changes the US would make in its policy toward Israel and Gaza.

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