The United States is sending three military aircraft to Egypt from Tuesday to bring vital humanitarian aid for Gaza during a truce between Israel and Hamas, senior US officials said.

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

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 region, which borders the Hamas-ruled Gaza Strip, into the stricken Palestinian territory itself, they said.

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

Mediator Qatar on Monday announced a 48-hour extension of an initial four-day truce, opening the way for further releases of hostages seized by Hamas during its attack on Israel.

- '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.

Hamas staged the deadliest attack in Israel's history when it broke through Gaza's militarized border on October 7. Israel says the attack killed 1,200 people, mostly civilians, and around 240 more were taken hostage.

In response, Israel launched a relentless bombing campaign and ground offensive in Gaza, which the Hamas government says has killed 15,000 people, thousands of them children.