WASHINGTON — The Biden administration warned that it could restrict weapons transfers to Israel if the flow of humanitarian aid into the Gaza Strip does not increase.

A letter sent Sunday by Secretary of State Antony Blinken and Defense Secretary Lloyd Austin accused Israel of sharply reducing humanitarian assistance to Gaza Palestinians. The letter also referred to a memorandum President Joe Biden issued in February linking military aid to how the receiving country administers humanitarian relief.

Israel was not named in the February memo but was widely seen as its target, at a time when the two governments were growing increasingly at odds over Israel’s conduct of the war.

“The amount of assistance entering Gaza in September was the lowest of any month during the past year,” the letter said, noting that the delivery of relief had improved considerably in the weeks immediately following Biden’s warning before decreasing again.

Giving Israel a 30-day deadline, the letter called for allowing in at least 350 trucks of relief a day, up from 69 per day in August, according to aid agencies. The letter also called for allowing people crowded in the coastal zone to move inward and for the removal of restrictions impeding the delivery of aid. The possible consequences outlined in the letter are broad, ranging from a second warning to suspending deliveries of weapons.

“Failure to demonstrate a sustained commitment to implementing and maintaining these measures may have implications for U.S. policy,” it said, referring to Biden’s national security memo. “Remediation could include actions from refreshing the assurances to suspending any further transfers of defense articles or, as appropriate, defense services.”

Democratic officials have consistently called on Israel to let more aid into Gaza. The letter was sent the same day that Vice President Kamala Harris, the Democratic presidential nominee, tweeted her concern at the slowdown in aid delivery.

“The U.N. reports that no food has entered northern Gaza in nearly 2 weeks,” Harris wrote. “Israel must urgently do more to facilitate the flow of aid to those in need. Civilians must be protected and must have access to food, water, and medicine. International humanitarian law must be respected.”

When weighing in on Israel and Gaza, Harris has sought to straddle competing Democratic constituencies that are both crucial to her election chances: Jewish and pro-Israel voters seeking assurances that she would continue Biden’s overall robust backing for the country in its multi-front war, and progressives and Arab and Muslim Americans and pro-Palestinian activists appalled at Israel’s conduct. She has been campaigning hard in Michigan, a swing state with what is believed to be the country’s largest Arab American population as well as a large Jewish voter base.

The letter outlined a number of obstacles currently impeding humanitarian assistance, including strict customs rules. And it said the vast majority of Gaza’s 2 million residents had been pushed into a small strip stretching from near the border with Egypt to less than halfway into the enclave, which posed additional risks.

“Extreme overcrowding has put these civilians at high risk of lethal contagion,” it said. “Humanitarian implementers report they are unable to meet essential survival needs of aid-dependent civilians.”

John Kirby, the National Security Council spokesman, said in a press call that the letter “follows a relatively recent decrease in humanitarian assistance reaching the people of Gaza, which is obviously something we’ve been very, very concerned about since the beginning of the conflict,”

The letter, he said, was spurred by “the sense of urgency that we all have here about the desperate need of the people of Gaza for this humanitarian assistance.” International health officials have long said that Gaza is in a famine or close to it.

Kirby would not predict what the breadth of the consequences would be should Israel be deemed non-compliant, but he reiterated the Biden administration’s commitment to assisting Israel in defending itself from Iran, noting the pending deployment of missile batteries and U.S. troops to Israel.

The deployment is “very much in keeping with the President’s strong desire that from an air defense perspective, our ally has what they need to defend themselves against, clearly, a very real, present, viable threat by Iran and its proxies,” he said.

An Israeli official, speaking on condition of anonymity to discuss a sensitive issue, indicated to the Jewish Telegraphic Agency that Israel would change its policies in response to the letter.

“The letter has been received and is being thoroughly reviewed by Israeli security officials,” the official said. “Israel takes this matter seriously and intends to address the concerns raised in this letter with our American counterparts.”

But Israel has also consistently denied that it is hindering aid delivery. On Monday, a day after the letter was sent but a day before it was leaked to the press, COGAT, the Israeli agency coordinating the delivery of goods into Gaza, tweeted video of what it said was 30 trucks entering the strip.

“Israel is not preventing the entry of humanitarian aid, with an emphasis on food, into Gaza,” it said.

Prime Minister Benjamin Netanyahu wants to crush Hamas in Gaza, even as the military is increasingly focused on fighting Hezbollah in Lebanon. On Monday, the Associated Press reported that Netanyahu was considering a proposal to give Palestinian civilians a week to leave northern Gaza before aid would be cut off entirely. Israeli Defense Minister Yoav Gallant reportedly told Austin that the proposal would not be implemented.

“In the last two weeks, the IDF has been conducting a ground operation in northern Gaza to destroy Hamas terrorist infrastructure, which just this week launched rockets from northern Gaza towards civilian populations in Israel,” COGAT said. “Israel will continue to allow the entry of humanitarian aid to the residents of Gaza, while simultaneously destroying Hamas’ military and governance infrastructures.”