In one of its most direct and sweeping warnings to date, the Biden administration told Israeli government officials on Sunday that if they did not improve humanitarian conditions in Gaza in the next 30 days, the U.S. could reevaluate its military support, which has flowed largely without pause for more than a year.

In a letter to Israel’s ministers of defense and strategic affairs, Secretary of State Antony Blinken and Secretary of Defense Lloyd J. Austin III said they were writing to “underscore the U.S. government’s deep concern over the deteriorating humanitarian situation in Gaza, and seek urgent and sustained actions by your government this month to reverse this trajectory.” The letter was first reported by Israeli media and confirmed Tuesday by the State Department’s top spokesperson.

Last month, ProPublica detailed how the U.S. government’s two foremost authorities on humanitarian assistance — the U.S. Agency for International Development and the State Department’s refugees bureau — concluded this past spring that Israel had deliberately blocked deliveries of food and medicine into Gaza. Those experts determined that weapons sales should be halted under a U.S. law, known as the Foreign Assistance Act, that requires the government to cut off military aid to a country that is blocking humanitarian efforts.

Blinken rejected their findings and, weeks later, told Congress that the State Department had concluded that Israel was not arbitrarily blocking aid. After the U.S. government raised concerns, the Israelis promised to allow more aid to flow.

Those pledges do not appear to have been met. According to Blinken and Austin’s letter, September was the worst month for relief efforts in the past year. The amount of aid has dropped by more than 50% since the spring. Israelis halted imports to Palestinian civilians, denied or impeded 90% of humanitarian movements between northern and southern Gaza last month, and imposed onerous new requirements for trucks carrying critical supplies, the letter says.

When asked about ProPublica’s previous reporting in September, Blinken told morning news programs he had evaluated input from several sources and made a decision that the Israelis weren’t deliberately blocking the aid. “We found that Israel needed to do a better job on the humanitarian assistance,” he allowed. “We’ve seen improvements since then. It’s still not sufficient.”

The State Department did not respond to ProPublica’s requests for comment Tuesday, but in a press conference, agency spokesperson Matthew Miller said that the letter was the latest effort to pressure the Israelis to address the crisis and that their improvements in the spring did not last. “The levels have not been sustained,” Miller said. “We are going to respond to events as they happen.”

“We know that it’s possible to get humanitarian assistance into Gaza,” he added.

Annelle Sheline, a former State Department official who resigned in protest earlier this year, said Sunday’s letter is a “clear acknowledgement that the administration knows” the Foreign Assistance Act is being violated. “This,” she added, “renders Israel ineligible to receive American weapons or security assistance.”

Responding to a similar criticism, Miller said, “We believe it’s appropriate to give them another 30 days to cure the problem.”

The humanitarian crisis has reached a critical stage, experts warn. The United Nations and other aid groups have become increasingly vocal about the deteriorating situation ahead of the winter. And an Al Jazeera documentary released in late September showed how Palestinian children had died of malnutrition.

On Monday, an Israeli military unit said it had allowed 30 trucks through a crossing in northern Gaza. “Israel is not preventing the entry of humanitarian aid, with an emphasis on food, into Gaza,” the unit wrote. “Israel will continue to allow the entry of humanitarian aid to the residents of Gaza, while simultaneously destroying Hamas’ military and governance infrastructures.” A spokesperson for the Israeli government did not immediately respond to requests for comment.

In the letter, Blinken and Austin make several specific demands of the Israelis, including allowing a minimum of 350 trucks per day through the four border crossings and opening up a fifth. They also warned the Israelis to not force civilians to evacuate from northern Gaza to the south.

“Failure to demonstrate a sustained commitment to implementing and maintaining these measures,” they wrote, “may have implications for U.S. policy.”

In addition, they called for a new channel to discuss civilian deaths. “Our engagements to date have not produced the necessary outcomes,” they wrote. At least 42,000 Palestinians have been killed by Israeli operations since Hamas’ Oct. 7 attack last year, according to the Gaza Health Ministry, an agency in the Hamas-controlled government.