U.S. Secretary of State Antony Blinken and Defense Secretary Lloyd Austin said Israel must take steps in the next month to improve the humanitarian situation in Gaza to avoid legal action involving U.S. military aid, according to news reports and sources.

"We are writing now 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," they wrote in an Oct. 13 letter to their Israeli counterparts, posted by an Axios reporter on X on Tuesday.

A reporter for Israeli News 12 first reported the contents of the letter on X.

Two sources familiar with the matter confirmed the letter's veracity to Reuters.

The State Department and Pentagon did not immediately respond to requests for comment on the letter. Representatives for Israel's government also could not be immediately reached for comment.

Washington has frequently pressed Israel to improve humanitarian conditions in Gaza since the war with Hamas began with the Palestinian militant group's attacks on southern Israel just over a year ago, but the Biden administration has not imposed restrictions on the military aid the United States sends to Israel.

The reports come as Israeli forces expand operations into northern Gaza amid ongoing concerns about access to humanitarian aid throughout the enclave and civilians' access to food, water and medicine.

Reuters reported earlier this month that food supplies have fallen sharply since Israeli authorities introduced a new customs rule on some humanitarian aid and are separately scaling down deliveries organized by businesses.

The United States told the U.N. Security Council last week that Israel needs to address urgently "catastrophic conditions" among Palestinian civilians in the besieged Gaza Strip and stop "intensifying suffering" by limiting aid deliveries.

The secretaries' letter outlined specific steps Israel must take within 30 days, including enabling a minimum of 350 trucks to enter Gaza per day, instituting pauses in fighting to allow aid delivery and rescinding evacuation orders to Palestinian civilians when there is no operational need.

"Failure to demonstrate a sustained commitment to implementing and maintaining these measures may have implications for U.S. policy ... and relevant U.S. law," the letter said.

It cited Section 620i of the Foreign Assistance Act, which prohibits military aid to countries that impede delivery of U.S. humanitarian assistance.

It also cited a National Security Memorandum U.S. President Joe Biden issued in February that requires the State Department to report to Congress on whether it finds credible Israel's assurances that its use of U.S. weapons does not violate U.S. or international law.

U.S. officials earlier this year said Israel may have violated international humanitarian law using U.S.-supplied weapons during its military operation in Gaza.