Joe Biden has called for an “immediate ceasefire” in Gaza, telling Benjamin Netanyahu that future US support for Israel will depend on it taking concrete action to protect civilians and aid workers. As the two leaders held their first phone call since Israeli airstrikes killed seven employees of the international food charity World Central Kitchen (WCK), Biden issued the strongest US rebuke toward Israel since the start of the conflict. In Thursday’s call, which lasted less than 30 minutes, the US president “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. “He made clear that US policy with respect to Gaza will be determined by our assessment of Israel’s immediate action on these steps.” Biden said that an “immediate ceasefire is essential” and urged Israel to reach a deal with Hamas “without delay”, the White House said. The statement marked a sharp change in Biden’s rhetoric and suggested, for what appears to be the first time, that strings could be attached to continued US support. The call came as WCK called on Australia, Canada, Poland, the US and the UK, whose citizens were killed in the attack, to join an independent investigation of the incident. “This was a military attack that involved multiple strikes and targeted three WCK vehicles,” the charity said in a statement. “All three vehicles were carrying civilians; they were marked as WCK vehicles; and their movements were in full compliance with Israeli authorities, who were aware of their itinerary, route and humanitarian mission. “An independent investigation is the only way to determine the truth of what happened, ensure transparency and accountability for those responsible, and prevent future attacks on humanitarian aid workers.” Biden’s comments were echoed by his secretary of state, Antony Blinken, who said US support would be curtailed if Israel failed to adjust its conduct. “If we don’t see the changes that we need to see, there will be changes in our policy,” he told reporters in Brussels. “Right now, there is no higher priority in Gaza than protecting civilians, surging humanitarian assistance and ensuring the security of those who provide it. Israel must meet this moment,” he said. The US has provided crucial military aid and diplomatic support for Israel’s nearly six-month offensive, which was launched in response to Hamas’s 7 October attack in southern Israel. Support for Israel’s ability to defend itself has been a touchstone of US foreign policy for more than half a century and until now there has been no indication the US was ready to condition – let alone withhold – military aid. The state department recently approved the transfer of 1,800 MK-84 2,000lb (907kg) bombs to Israel, with the decision reportedly coming on the day of the WCK strike. The national security council spokesperson, John Kirby, on Thursday reiterated that US support for Israel’s defence remained “iron-clad”. But earlier on Thursday a key Biden ally urged the US president to use the leverage afforded by the aid. “I think we’re at that point,” Chris Coons, a Democratic senator from the president’s home state of Delaware, told CNN. Coons said that if Israel began a long-threatened offensive in the southern city of Rafah, without plans for the 1.5 million people sheltering there, “I would vote to condition aid to Israel”. Biden’s comments were the latest in a chorus of international anger over the attack on the WCK team, which was distributing desperately needed food to a population facing famine. The Australian prime minister, Anthony Albanese, said on Thursday that Israel’s explanation for the deaths was “not good enough”, while a diplomatic crisis between Poland and Israel has erupted after Israel’s ambassador to Poland, Yacov Livne, pushed back at what he said were attempts by the “extreme right and left in Poland” to accuse Israel of “intentional murder”. The Polish president, Andrzej Duda, on Thursday called the comment “outrageous” and described the ambassador as “the biggest problem for the state of Israel in relations with Poland”. The Polish foreign ministry said Livne was summoned to a meeting on Friday morning. Israel said on Thursday that its military investigation into the killing of the seven aid workers could take weeks, while Shimon Freedman, a spokesperson for Cogat, the arm of the Israeli military responsible for civilian affairs in the Palestinian territories, told the BBC they “hope for [the investigation to report] in the next few days”. The IDF chief of staff, Herzi Halevi, attributed the strike to “misidentification”, adding that it “was not carried out with the intention of harming WCK aid workers”, and was a mistake that should not have happened. The IDF said it had halted leave for all combat units on Thursday and heightened its air defence command to deal with a possible missile or drone attack from Iran. There is concern in Israel about Tehran’s response to the deaths of two Iranian generals and five military advisers in an Israeli airstrike on an Iranian diplomatic compound in Damascus earlier this week. Netanyahu has faced intense domestic pressure from the families and supporters of the hostages still being held in Gaza, and from a resurgent anti-government protest movement. On Wednesday Benny Gantz, a Netanyahu rival and member of the war cabinet, called for snap parliamentary elections in September. “We must set a consensual date for the month of September, or if you prefer for the first anniversary of the war,” Gantz said. The prime minister’s Likud party rejected the call, but it was welcomed by the leader of the US Senate, Chuck Schumer, who last month urged new elections in a strident criticism of Netanyahu’s handling of the conflict. “When a leading member of Israel’s war cabinet calls for early elections and over 70% of the Israeli population agrees according to a major poll, you know it’s the right thing to do,” Schumer tweeted. Early elections require the agreement of 61 elected officials, or the majority of deputies in the Knesset, where the Likud has the most seats but does not have a majority. Likud said a national election while Israel was at war “would inevitably lead to paralysis” and harm the military’s fight in Gaza. Meanwhile, Hamas official Osama Hamdan said there had been no progress in Gaza ceasefire talks despite the Palestinian group showing flexibility. “Negotiations are stuck in a vicious circle,” Hamdan said. More than 33,000 Palestinians have been killed in the war, Gazan medical officials say. Hamas has said 6,000 of its fighters are among the fatalities. Israeli officials say the Palestinian combatant death toll is more than twice that number.