The US Senate narrowly confirmed former Fox News co-host Pete Hegseth as Pentagon chief on Friday, despite allegations of alcohol abuse, sexual misconduct and other fears about his ability to lead the world's most powerful military.

Three Republican senators voted against Donald Trump's pick as secretary of defense, resulting in a 50-50 tie that required JD Vance to cast the deciding ballot – only the second time in history a vice president has had to intervene to save a cabinet nominee.

The razor-edged result underscored concerns about Hegseth, who will take over the Pentagon with war raging in Ukraine, the Middle East volatile despite ceasefires in Lebanon and Gaza, and as Trump expands the military's role in security on the US-Mexico border.

The 44-year-old is a former Army National Guard officer who until recently worked as a co-host for Fox News – one of Trump's favoured television channels.

Hegseth has a combative media personality, fierce loyalty and telegenic looks – common hallmarks in Trump's entourage.

Supporters say Hegseth's deployments in Afghanistan and Iraq give him the insight to run the Defense Department better than more experienced officials who would normally be considered for the job.

He has pledged to focus the military on "lethality" and to bring back "warrior culture" to the Pentagon.

In confirming him, Republicans brushed aside his lack of experience leading an organisation anywhere near the size of the Defense Department – the country's largest employer with some three million personnel.

They also approved Hegseth despite allegations of financial mismanagement at veterans' nonprofits where he previously worked, reports of excessive drinking, and allegations that he sexually assaulted a woman in California.

Asked during his confirmation hearing last week about criticism he has faced, Hegseth said there was a "coordinated smear campaign" against him, and that he is "not a perfect person, but redemption is real".

Trump has stood by him, telling reporters on Friday: "Pete's a very, very good man."

Shortly after he was confirmed, Trump wrote on his Social Truth platform: "Congratulations to Pete Hegseth. He will make a great Secretary of Defense!" (AFP)