Defense Secretary Pete Hegseth told the Pentagon to develop the deep reductions, The Washington Post reported, citing a memo.

The Pentagon's budget for 2025 is some $850 billion. Lawmakers across the political spectrum agree that the massive spending is needed to deter threats, especially from China and Russia.

The cuts, if implemented in full, would reduce that figure by tens of billions each year to some $560 billion by the end of the five years.

The report did not give details of where the cuts would be made in the world's biggest military, but an earlier Post report said that junior civilian workers, not uniformed personnel, were being targeted.

The news -- which comes after Elon Musk's Department of Government Efficiency reportedly visited the Pentagon last week -- was likely to be met with stiff resistance from both the military and Congress.

Trump on Wednesday signaled support for a House of Representatives bill that would increase the defense budget by $100 billion -- a move at odds with the Hegseth-directed cuts.

The planned reductions also run counter to calls by Trump and Hegseth for NATO members to increase their military spending to five percent of GDP a year.

- 'Revive the warrior ethos' -

The United States currently spends around 3.4 percent of its GDP on defense, and the five percent threshold would be even farther out of reach if the Pentagon's budget is reduced.

The stock prices of major US defense contractors were hit by the news, with Lockheed Martin dropping briefly before recovering, Northrop Grumann falling nearly two percent and Palantir closing down more than 10 percent.

Hegseth's memo said the proposed cuts must be drawn up by February 24, and include 17 categories that Trump wants exempted, including operations at the US border with Mexico and modernization of nuclear weapons and missile defense.

It also calls for funding for regional headquarters such as Indo-Pacific Command and Space Command.

But other major centers such as European Command, which has led the way on US strategy throughout the war in Ukraine, and also Africa Command and Central Command -- which oversees operations in the Middle East -- were absent from the list, the Post reported.

The Defense Department "must act urgently to revive the warrior ethos, rebuild our military, and reestablish deterrence," Hegseth wrote in the memo, dated Tuesday, according to the Post.

"Our budget will resource the fighting force we need, cease unnecessary defense spending, reject excessive bureaucracy, and drive actionable reform including progress on the audit," he reportedly continued.

US President Donald Trump has vowed to slash government spending and end US support for Ukraine in its war against Russian invasion.