Spanish celebrity chef Jose ­Andres, whose aid workers including an Australian were killed by ­Israeli forces in Gaza last week, has blasted Israel’s military operations in the besieged enclave as a “war against humanity itself”, while Americans’ support for its once close ally, including that of Donald Trump, appears to crumble.

As the six-month mark of the bloody conflict ticked over, Andres said the deaths of the seven World Central Kitchen workers at the hands of Israel ­Defence Forces was “unforgivable” and cast doubt on the integrity of Israel’s recently completed investigation.

“I will say something – so complicated – the investigation should be much deeper,” Andres told US ABC News in an interview that aired on Monday AEST, ­referring to Israel’s apology for the deaths and its subsequent sacking of two ­officers who it said had flouted protocol.

“And I would say that the perpetrator cannot be investigating himself,” he added, in remarks that will intensify pressure on the White House to shift its unconditional backing of Israel, whose military operation to destroy Hamas has reportedly killed more than 33,000 Palestinians, according to Hamas health authorities.

US national security spokesman John Kirby also told ABC the White House was “increasingly frustrated … with the way these ­operations are being prosecuted and the way that Israelis are acting on the ground in terms of civilian casualties”.

President Joe Biden, following a deluge of international condemnation of the deaths Israel said were a terrible mistake, told Israeli Prime Minister Benjamin Netanyahu in a phone call earlier this week to do more to facilitate the safe transport of food into Gaza, fuelling expectations the US would impose conditions on its ally.

The change in Washington’s stance, after initially offering steadfast support following terrorist ­attacks that killed about 1200 Israelis, comes amid a marked ­decline in American support for ­Israel, including among Republicans, whose evangelic Christian base had typically offered the greatest support.

Before the aid workers’ deaths, Americans’ approval of Israel’s ­actions in Gaza had dropped to 36 per cent in late March from 50 per cent in November, based on a Gallup poll conducted nationally in the first three weeks of March.

Among Republican voters, support had fallen to 64 per cent from 71 per cent in November, alongside bigger falls among independent and Democratic Party voters, where support has fallen from 47 to 29 per cent, and 36 to 18 per cent. The share of voters who disapproved rose above half, to 55 per cent, for the first time.

Mr Trump, whose administration was a strong backer of Israel, overseeing the shift of the US ­embassy from Tel Aviv to Jerusalem, shocked some observers last week after he refused in an interview with US broadcaster Hugh Hewitt to declare he was “100 per cent” behind Israel.

“They’ve got to finish what they started, and they’ve got to finish it fast, and we have to get on with life,” Mr Trump said, criticising the IDF for releasing so many images and videos of Gaza’s devastated ­infrastructure.

“I guess it makes them look tough,” he said. “But to me, it doesn’t make them look tough. They’re losing the PR war. They’re losing it big.”

The former president had said nothing publicly about Israel’s military response, apart from criticising Mr Netanyahu for an alleged intelligence failure in the days immediately after October’s terrorist attack, until the past few weeks.

The slump in support for Israel among Democrats could imperil the passage of a $US95bn foreign aid bill passed by the Senate in February, which included about $US17bn in aid for Israel and $US60bn for Ukraine – a bill ­expected, in some form, to face a vote in the House of Representatives this week.

Influential former speaker Nancy Pelosi and 36 other Democratic congressmen in a public letter released at the weekend urged Mr Biden to halt arms transfers to Israel, potentially jeopardising passage of the controversial omnibus aid package that Democrats had championed.

“Is that a serious question?” the President replied when asked by a reporter if the US was abandoning Israel.