The White House has just published an official record of the call between the president Joe Biden and Israeli PM Benjamin Netanyahu, BBC reported.
It says the two leaders discussed the situation in Gaza, with Biden "emphasising that the strikes on humanitarian workers and the overall humanitarian situation are unacceptable".
Biden is also said to have 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 readout adds that Biden told his Israeli counterpart, the US's policy "with respect to Gaza will be determined by our assessment of Israel’s immediate action on these steps" - and stressed that an immediate ceasefire "is essential to stabilize and improve the humanitarian situation and protect innocent civilians".
Biden had earlier said he was "outraged and heartbroken" at Israel's killing of the World Central Kitchen employees.