US Secretary of State Antony Blinken and Defense Secretary Lloyd Austin wrote a letter to Israel on Sunday to make clear Washington's concerns about the levels of humanitarian assistance that have been arriving to Gaza, the State Department said on Tuesday.

State Department spokesperson Matthew Miller told reporters at a regular news briefing that the US knows it is possible to get humanitarian aid into Gaza and that bureaucratic and logistical obstacles can be surmounted.

"We need to see further changes by the government of Israel," Miller said, adding that there are implications under US law and Washington hopes that Israel will make the changes outlined in the letter.

The Pentagon said on Tuesday a letter co-authored by US Defense Secretary Lloyd Austin to Israeli officials about the humanitarian situation in Gaza was "private correspondence" and declined to discuss it in detail.