Washington DC - President Joe Biden warned Prime Minister Benjamin Netanyahu Thursday that US policy on Israel depends on the protection of civilians in Gaza, in his strongest hint yet of possible conditions on military aid after an Israeli strike killed seven aid workers.

In their first call since the deaths of the employees of the US-based World Central Kitchen group on Monday, Biden also called for an "immediate ceasefire" after the "unacceptable" attack and wider humanitarian situation in Gaza.

Democrat Biden is facing growing pressure in an election year over his support for Israel's Gaza war – with allies pressing him to consider making the billions of dollars in military aid sent by the US to its key ally each year dependent on Netanyahu listening to calls for restraint.

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 readout of the call.

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"He made clear that US policy with respect to Gaza will be determined by our assessment of Israel's immediate action on these steps."

National Security Council spokesman John Kirby said that the US wanted to see action within the "coming hours and days," including a "dramatic" increase in aid to Gaza, where the US has warned of impending famine.

Kirby acknowledged that the call followed "growing frustration" with Netanyahu, although he reiterated that US support for Israel's security was "ironclad."

A key Biden confidant had earlier urged him to use the leverage afforded by the huge military aid that Washington gives Israel – something Biden has resisted for the past six months.

"I think we're at that point," Chris Coons, a Democratic senator from the president's home state of Delaware, told CNN.

If Israel began its long-threatened full-scale offensive in the southern city of Rafah without plans to protect some 1.5 million people sheltering there, "I would vote to condition aid to Israel," Coons said.

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"I've never said that before, I've never been there before," he added.

Biden also reportedly faces pressure from even closer to home – from First Lady Jill Biden.

"Stop it, stop it now," she reportedly told the president about the growing toll of civilian casualties in Gaza, according to comments by Biden himself to a guest during a meeting with members of the Muslim community at the White House.