Washington DC - The United States on Tuesday sent the first of three military planes to Egypt to bring vital humanitarian aid for Gaza during a truce between Israel and Hamas, 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.

The first Air Force C-17 aircraft landed Tuesday in Egypt with 54,000 pounds of medical supplies and ready-to-eat food, the US Agency for International Development said.

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"With 1.7 million people internally displaced and 2.2 million in need of humanitarian assistance, increased humanitarian supplies are essential to saving lives and alleviating suffering for the most vulnerable," USAID said in a statement.

The United Nations will take the aid from Egypt's North Sinai region, which borders the Hamas-ruled Gaza Strip, into the stricken Palestinian territory itself, officials said. Two further planeloads will arrive in the 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.

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 senior US official told reporters on an embargoed call Monday.

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.

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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," another official said.