The Biden administration has warned Israel that it faces possible punishment, including the potential stopping of US weapons transfers, if it does not take immediate action to let more humanitarian aid into Gaza.

A letter written jointly by Antony Blinken, the US secretary of state, and Lloyd Austin, the defence secretary, exhorts Benjamin Netanyahu’s government to ease humanitarian suffering in the territory by lifting restrictions on the entry of assistance within 30 days or face unspecified policy “implications”.

The four-page missive, dated 13 October, was sent to Yoav Gallant, the Israeli defence minister, and Ron Dermer, the strategic affairs minister, and came to light after being posted on social media by Barak Ravid, an Israeli journalist who works for Axios, after apparently being leaked.

Its authenticity was confirmed by a state department spokesperson, Matthew Miller, at a news briefing on Tuesday.

Humanitarian groups have made repeated calls for increased deliveries of food and medicine to Gaza, but aid shipments to the embattled territory are currently at their lowest level in months, the UN said last week.

An Israeli official in Washington told Reuters that Israel was reviewing the letter.

“Israel takes this matter seriously and intends to address the concerns raised in this letter with our American counterparts,” the official said.

Miller said the US side had intended the letter to be a private diplomatic communication and said its timing was not influenced by next month’s presidential election, which features a knife-edge contest in the battleground state of Michigan, where many Arab American voters have voiced anger over the White House’s support for Israel’s conduct of the war.

Democrat strategists harbour fears that discontent over Gaza could result in Kamala Harris, the vice-president and party nominee, losing the state to Donald Trump in the 5 November poll.

The letter complains of delays to US-funded aid at crossing points into Gaza and says the flow of assistance into the war-devastated territory has dropped by more than 50% since Israel promised last March to allow more deliveries.

“We are particularly concerned that recent actions by the Israeli government … are contributing to an accelerated deterioration in the conditions in Gaza,” it says.

White House national security spokesperson John Kirby said that the letter was not intended as a threat, but “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”.

After an uptick in assistance after communications between the US and Israel in March and April, aid volumes entering the strip in September fell to their lowest level, Blinken and Austin wrote, since last October, when Israel launched a massive military offensive in retaliation for an attack by Hamas that killed about 1,200 Israelis, and led to more than 250 being taken hostage.

“To reverse the downward humanitarian trajectory and consistent with its assurances to us, Israel must, starting now and within 30 days, act” on a series of specific steps, including letting in at least 350 aid trucks daily and instituting humanitarian pauses to Israeli military activity.

The letter adds: “Failure to demonstrate a sustained commitment to implementing and maintaining these measure may have implications for US policy under NSM-20 and relevant US law.”

NSM-20 refers to a memorandum issued by the White House national security council, which allows for “appropriate next steps” if a country receiving US military aid is deemed by the state department or the Pentagon not to be meeting prior assurances of allowing the delivery of humanitarian assistance.

“Such remediation could include actions from refreshing the assurances to suspending any further transfers of defense articles or, as appropriate, defense services,” the memorandum states.

Congressional Republicans have called on the White House to revoke NSM-20 calling it “redundant” and dismissing it as aimed at “placat[ing] critics of security assistance to our vital ally Israel”.

Other relevant legislation that could be invoked include section 620I of the Foreign Assistance Act and the Leahy Act, which preclude the US government from providing military assistance or selling arms to countries that restrict humanitarian aid or violate human rights.

Miller, the state department spokesperson, declined to go into specific when asked what consequences Israel might face for refusing to meet American demands for greater aid access.

He said that a previous letter Blinken had written in April had increased humanitarian aid flows. An Israeli official confirmed that the latest letter had been received but did not discuss the details, the Associated Press reported.

Miller also said that Blinken had seen footage showing at least one Palestinian burned alive after an Israeli strike set tents ablaze outside a Gaza hospital.

“We all saw that video, and all know that it’s horrifying to see people burned to death. We have made clear our serious concerns about the matter directly with the government of Israel.”

The US has made repeated exhortations to allow increased aid into the te, but Netanyahu has frequently ignored such entreaties to moderate its conduct of the war in Gaza.

Last week, UN spokesperson Stéphane Dujarric said that the three hospitals still operating in northern Gaza face “dire shortages” of fuel, medicine and blood, while food supplies are dwindling.

Israeli authorities facilitated just one of 54 UN attempts to get aid to north Gaza this month, Dujarric said. Eighty-five percent of the requests were denied, with the rest impeded or canceled for logistical or security reasons.

Israel insists that much of the aid has dual-use capacity that could help Hamas fighters and also says it has been subject to looting.

More than 42,000 Palestinians have been killed and the majority of buildings in Gaza destroyed or badly damaged in Israel’s year-long offensive with the stated aim of rooting out Hamas.

The Pentagon described the letter as “private correspondence” and declined to discuss it in detail.