Their missive underscores the extreme friction surrounding America’s ties with the Jewish state, a relationship characterised by close co-ordination amid the Israeli military’s ongoing war against Hamas, the Iranian-backed militant group whose fighters attacked the country on October 7, 2023, but also intense strains over Israel’s conduct of that fight.

It comes, too, at a moment of heightened fear in Washington about mounting instability across the Middle East, as Israel expands a new campaign against the Iranian-backed militant group Hezbollah in Lebanon and the Pentagon deploys US troops and air defence equipment to Israel amid the country’s heightened tensions with regional powerhouse Tehran.

“We’re looking to see concrete measures taken to address the humanitarian situation in Gaza, which we know continues to be an issue,” the Pentagon’s deputy press secretary, Sabrina Singh, told reporters. “We want to see … that they’re considering civilians in the battle space, and that’s what we’ve said from the beginning.”

Netanyahu’s office did not immediately respond to a request for comment. The United States is by far Israel’s largest external backer, providing the country $3.8 billion a year in military aid.

Critics of President Joe Biden’s handling of the conflict in Gaza – where more than 40,000 people have died in the past year of conflict, according to local health authorities – have blamed him for not effectively leveraging American support to convince Israel to comply with US demands for an end to the violence in Gaza and a steady flow of food and other aid supplies.

Since the war began, Biden has halted only one shipment of US weapons bound for Israel. In May, the President decided to continue the flow of arms despite an administration review which found that Israel had contributed significantly to the insufficient flow of aid to Palestinian civilians.

The letter follows a similar message Blinken sent the Israeli Government in April, which officials said resulted in a temporary improvement in aid flows into the battered Strip. But the picture in Gaza has darkened once more in recent weeks, as Israeli officials have returned to areas where they cite the resilience of Hamas, particularly in the battle-ravaged northern areas of the Strip.

According to the UN Office for the Coordination of Humanitarian Affairs (Ocha), no food has entered northern Gaza since early October, increasing the risk of famine amid what it has described as an Israeli “siege” of the area. Residents who have been displaced multiple times have fled parts of northern Gaza once more in the last week amid a renewed Israeli offensive in the area.

The Israeli military office that oversees aid distribution in Gaza, Cogat, has denied that Israel has halted the entry or coordination of aid in northern Gaza. Israeli officials have acknowledged shortfalls in the delivery of aid since the beginning of the conflict but primarily blame Hamas and criminal gangs for the problems.

Biden made his displeasure with the situation in northern Gaza known to Netanyahu when the two leaders spoke last week, noting “the imperative to restore access to the north, including by reinvigorating the corridor from Jordan immediately,” according to the White House.

The war has emerged as a major political liability for Biden, whose Democratic Party is now split over US support to Israel. It remains unclear how the war will impact the chances of Biden’s Vice-President, Democratic nominee Kamala Harris, when she faces off with former President Donald Trump in November’s election.

Trump’s Republican Party has been seeking to capitalise on Harris’ vulnerability with Arab and Muslim voters because of the bloodshed in Gaza.

In their letter, Austin and Blinken laid out specific criteria the Biden administration would like to see within the next month, including ensuring the entry of at least 350 aid trucks a day into Gaza; putting in place humanitarian pauses to enable aid delivery; lifting evacuation orders when they are no longer needed; enabling aid shipments from Jordan; and halting the isolation of northern Gaza, according to officials who spoke on the condition of anonymity to address the contents of the letter.

Austin has spoken more than 80 times with Gallant since October 7, repeatedly urging him to reduce civilian casualties in Gaza and ensure the facilitation of humanitarian aid.

News of Blinken and Austin’s letter emerged just hours after the Pentagon confirmed the arrival of American troops and equipment in Israel that are part of a new deployment of one of its most advanced missile defence systems there. Approximately 100 troops are eventually expected to be positioned in Israel to operate the Terminal High Altitude Area Defense (Thaad) missile system.

The system is intended to bolster Israel’s already robust air defence network as the country prepares to conduct a counterstrike following Iran’s ballistic missile barrage on Israel this month. The Biden administration has urged Israel to conduct only a limited response to avoid further deterioration of regional security.

While Biden administration officials on Tuesday made clear their frustration with the humanitarian situation in Gaza as well as the heavy civilian death toll of Israel’s recent operations in Lebanon, they repeatedly avoided, under questioning from reporters, making explicit threats about cutting off the flow of US military aid, saying instead that they hoped Israeli leaders would make the changes demanded in the letter.

State Department spokesman Matthew Miller told reporters that levels of humanitarian aid were 50% lower now than it was at its peak. However, he declined to detail what would happen if Israel did not comply with the US requests.

“Ultimately, what we want to see here is results,” he told reporters. “Our hope is that Israel will make the changes that we have outlined and that we have recommended, and that the result of those changes will be a dramatic increase in humanitarian assistance.”

White House National Security Council spokesman John Kirby said the letter “was not meant as a threat”.

“The letter was simply meant to reiterate the sense of urgency we feel and the seriousness with which we feel it about the need for an increase, a dramatic increase in humanitarian assistance,” he said in a phone briefing with reporters. “That’s what you can do with your ally. And it’s not the first time we’ve, we’ve communicated that to Israel. But hopefully, we won’t have to communicate it again.”

Asked why the Biden administration did not simply cut off aid to Israel immediately, given Netanyahu’s repeated defiance about the concerns about the civilian toll, Miller said it was “appropriate to give them some time to work through the different issues and find ways to get the level of trucks, get the level of food, water, medicine, back up to acceptable levels”.

US officials said they believe giving Israel 30 days to comply with the demands – versus forcing a faster schedule – was reasonable, given the complexity of the response required and the schedule of US weapons deliveries.

But the administration’s reluctance to publicly confirm that it would restrict arms shipments fuelled immediate doubts about the seriousness of the warning from Blinken and Austin.

Jeremy Konyndyk, the president of Refugees International and a former senior Biden administration official, said he expected that Netanyahu, who has repeatedly defied US requests in the conduct of his war, would be sceptical that Biden had put “real teeth” behind this warning.

“If it is truly different this time – and I hope it is – the White House will need to make that much more explicit,” Konyndyk said. “Otherwise, this will have no more effect than all the warnings he has previously ignored.”