Washington DC - The US Senate approved Donald Trump's nominee to be the nation's top military officer on Friday after the president abruptly fired the previous chairman of the Joint Chiefs of Staff this year.

Lawmakers signed off on Dan Caine's nomination and promotion to four-star general in an early morning vote of 60 to 25 ahead of a two-week recess.

Trump's administration has dismissed a series of senior officers as part of a rare and major shakeup of top US military leadership that began shortly after he returned to office in January.

Democrats have sharply criticized the firings – including of the previous Joint Chiefs chairman, General Charles "CQ" Brown – accusing Trump and Defense Secretary Pete Hegseth of seeking to ensure the military is led by people loyal to the president.

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Caine sought to allay such concerns about his nomination during his confirmation hearing last week, pledging that he would "continue the traditions and standards of my oath of office and my commission as a nonpartisan leader who will always strive to do the right thing."

He said that guarding against politicization of the military "starts with being a good example from the top and making sure that we are nonpartisan and apolitical and speaking the truth to power every day."

"The nation and the Constitution all require a nonpartisan military," Caine said.

The president fired Brown in February, less than two years into his four-year term as chairman, which began under Trump's predecessor Joe Biden – a move that cleared the way for Caine's nomination.

Other senior officers dismissed this year include the heads of the Navy and Coast Guard, the general who headed the National Security Agency, the vice chief of staff of the Air Force, a Navy admiral assigned to NATO, and three top military lawyers.

Hegseth has insisted that Trump is simply choosing the leaders he wants, but Democratic lawmakers have raised concerns about the potential politicization of the US military.