The Biden administration has warned Israel that it must increase the amount of humanitarian aid it is allowing into Gaza within the next 30 days or it could risk losing access to U.S. weapons funding.

Secretary of State Antony Blinken and Defense Secretary Lloyd Austin warned their Israeli counterparts in a letter dated Sunday that the changes must occur. The letter, which restates U.S. policy toward humanitarian aid and arms transfers, was sent amid deteriorating conditions in northern Gaza and reports Israel had conducted a strike on a hospital tent site in central Gaza that killed at least four people.

A senior defense official said Tuesday that Blinken and Austin sent a letter to their Israeli counterparts as they saw a recent decrease in assistance reaching Gaza. The official said a similar letter sent by Blinken in April triggered a constructive response and “concrete measures from the Israelis.”

The official, who spoke on condition of anonymity to discuss internal deliberations, said the latest letter was a way to similarly address the problem.

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For Israel to continue qualifying for foreign military financing, the level of aid getting into Gaza must increase to at least 350 trucks a day, Israel must institute additional humanitarian pauses and provide increased security for humanitarian sites, Austin and Blinken said. They said Israel had 30 days to respond to the different requirements.

An Israeli official confirmed a letter had been delivered but did not discuss the contents. The official, speaking on condition of anonymity to discuss a diplomatic matter, confirmed the U.S. had raised “humanitarian concerns” and was putting pressure on Israel to speed up the flow of aid into Gaza.