The US has issued a formal warning to Israel, giving the country a 30-day deadline to improve humanitarian aid access to Gaza or face potential cuts to US military assistance, according to reports. The letter, sent on Sunday, marks the strongest known written warning from the US to its ally as civilian casualties rise amid Israel’s new offensive in northern Gaza.

The letter expresses deep concerns about the worsening humanitarian crisis in Gaza, stating that Israel obstructed nearly 90% of humanitarian movements between the northern and southern parts of the territory last month.

An Israeli official said the country is reviewing the letter, adding that Israel “takes this matter seriously” and plans to “address the concerns raised” with US officials.

Israel has previously stated it is focusing on targeting Hamas operatives in the north while not preventing humanitarian aid from entering.

On Monday, Cogat, the Israeli military body managing Gaza crossings, reported that 30 aid trucks from the World Food Programme entered northern Gaza through the Erez crossing. This marked the first food aid delivery in two weeks, with the UN warning that essential supplies were running out for the 400,000 Palestinians in the north.

A UN official described Gaza as being in a state of “constant peak emergency,” with people in the north “relying solely on assistance” and facing severe shortages of fresh food except for what is provided by UN agencies.