Biden issues toughest warning yet to Israels' Netanyahu

President Joe Biden warned Israel of a sharp shift in his policy over the Gaza war Thursday amid growing frustration with Benjamin Netanyahu and mounting domestic pressure in a US election year.

A tense phone call between the two leaders appeared to yield at least some results, as Netanyahu's office announced within hours it would temporarily allow more aid to flow into Gaza.

Biden has stood solidly behind Israel and its prime minister since Hamas's October 7 attacks, and his criticism of civilian deaths in Gaza has not stopped Washington from supplying military hardware to its key ally.

But the Democrat faces spiralling anger from Muslim and younger voters over his support for Israel, and political allies have been pressing him to make the aid conditional on changed Israeli behaviour.

In a 30-minute call with Netanyahu after Israeli strikes killed seven aid workers from US-based charity World Central Kitchen, Biden hinted at doing just that.

For the first time, Biden "made clear that US policy with respect to Gaza will be determined by our assessment of Israel's immediate action" to curtail the killing of civilians and aid workers and improve the dire humanitarian situation, the White House said.

The tough language, describing the aid worker attack as "unacceptable" and urging Israel to take steps towards an immediate ceasefire, reflected mounting tensions with Netanyahu.

"What happened to the World Central Kitchen, that event certainly was a catalyst for the call today," White House National Security Council spokesman John Kirby told CNN Thursday evening.

Earlier this week the WCK convoy was repeatedly struck by Israeli forces despite clearing its route with the military. A US-Canadian dual national was among those killed in the attack, which Israel has called a mistake.

Biden's "frustration has been growing over recent weeks and months over the dangers to the civilian population in Gaza and to aid workers," Kirby said.

Later that evening Netanyahu's office announced it was reopening the Erez border crossing for the first time since October 7 to allow aid into the northern Gaza Strip, as well as allowing supplies to pass through the port of Ashdod. Increased levels of Jordanian aid will go through Kerem Shalom.

The White House said the steps were taken "at the president's request" and "must now be fully and rapidly implemented."

The Biden administration faces a difficult balancing act on Gaza between what Kirby called its overall "ironclad" commitment to Israel and mounting opposition at home to the war.

Democrats fear anger among Arab American and Muslim voters in an expected tight election against Republican Donald Trump in November.

A key Biden confidant had earlier urged him to leverage the huge military aid that Washington gives Israel every year.

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

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

"Stop it, stop it now," she told the president about the growing toll of civilian casualties in Gaza, according to comments by Biden himself during a meeting with members of the Muslim community, reported by the New York Times.

Biden ordered US airdrops of aid into Gaza. He also allowed a UN Security Council resolution calling for a ceasefire to go through -- infuriating Netanyahu.

Biden's words, however, have not been matched by concrete steps to limit the billions of dollars in military aid flowing to Washington's bedrock regional ally.

The United States approved the transfer of thousands more bombs to Israel on the same day as the Israeli strikes that killed the WCK aid workers, the Washington Post reported Thursday.

US voters are increasingly turning against Israel's Gaza offensive.

A majority of 55 per cent now disapprove of Israel's actions, compared to 36 per cent who approve, according to a Gallup poll released on March 27.