The relief flights carrying food, medical supplies, and winter gear are the first by the US military since the conflict began on October 7.

The flights start a day after President Joe Biden said he would use an extension of the truce to get more aid into Gaza, and as international efforts continue to further prolong the pause.

"We are very glad to announce we will have the first of three relief flights that are facilitated by the unique capabilities of the US military that will be arriving in North Sinai in Egypt" on Tuesday, one of the US officials said.

"This will be to bring a series of items -- medical items, food aid, winter items, given that winter's coming in Gaza -- for the civilian population," the official said in an embargoed call on Monday.

The United Nations will then take the aid from Egypt's North Sinai to the Gaza Strip, they said.

Two further planeloads will arrive "in coming days", they added.

On Monday, a 48-hour extension of an initial four-day truce between Israel and Hamas - mediated by Qatar, Egypt, and the US - was announced, opening the way for further releases of captives seized by Hamas on 7 October and Palestinian prisoners from Israeli hails.

'Significant surge' 
 

Eight hundred aid trucks reached southern Gaza from Egypt in the first four days of the truce, with some aid also reaching badly hit northern Gaza, the US officials said.

"The movement over the last four or five days of assistance has been so significant in volume that a backfill... is now needed and these planes are part of that backfill," a second US official said.

While Washington has deployed two aircraft carriers in the region to deter Iran and its allies, and ferried military assistance to key ally Israel, it has not previously used military assets during this conflict to deliver humanitarian aid.

Biden, who has firmly backed Israel while calling on it to reduce civilian casualties, said on Monday that the truce had allowed a "significant surge" in aid.

The White House said on Monday however that Israel had made it clear it would continue its war on Hamas whenever the truce ended.

US officials said Biden had warned Israel that it must not cause the same kind of mass displacements in southern Gaza that its offensive in the north triggered earlier this month.

"From the president down we have reinforced this in a very clear way for the government of Israel," the first US official said.