The United States has warned Israel to increase the flow of humanitarian supplies into the war-devastated Gaza Strip within the next 30 days or risk losing military aid, American officials said Tuesday.

The warning came in a letter signed by the American secretaries of defense and state that was sent on Sunday to Israel’s defense minister and its minister of strategic affairs. It was confirmed on Tuesday by a State Department spokesman, Matthew Miller.

Mr. Miller said the amount of aid entering Gaza in September was the lowest it had been at any time since the Hamas attack on Israel on Oct. 7, 2023, that set off the Israeli invasion.

“What we have seen over the past few months is that the level of humanitarian assistance has not been sustained,” Mr. Miller told reporters in Washington. “In fact, it has fallen by over 50 percent from where it was at its peak.”

The warning came as the Israeli government told the Biden administration that it would not strike Iran’s nuclear enrichment and oil production sites when it responds to Tehran’s recent missile attack on Israel, officials said. That concession may reduce the immediate likelihood of an all-out war between the two adversaries.

The officials, who spoke on the condition of anonymity to discuss sensitive diplomacy, said that Israel had agreed to focus its next attack on military targets in Iran. The Biden administration believes that if Israel were to hit oil or uranium enrichment sites, it could set off a dramatic escalation of Middle East hostilities at a time when Israel is already at war with Iran’s regional proxies, including Hamas in Gaza and Hezbollah in Lebanon.