Vice President JD Vance has broken the Senate's 50-50 vote to confirm Pete Hegseth as the US Secretary of Defense despite questions over qualifications and allegations of heavy drinking and aggressive behaviour toward women.

The voting late on Friday was at a 50-50 tie before Vance broke the tie.

Rarely has a Cabinet nominee faced such wide-ranging concerns about his experience and behaviour as Hegseth, particularly for such a high-profile role leading the US military.

But the Republican-led Senate was determined to confirm Hegseth, a former Fox News host and combat veteran, to deliver President Donald Trump's and round out his top national security Cabinet officials.

Senate Majority Leader John Thune opened Friday's session by saying that Hegseth, as a veteran of the Army National Guard who served tours in Iraq and Afghanistan, "will bring a warrior's perspective" to the top military job.

"Gone will be the days of woke distractions," Thune said, referring to the diversity, equity and inclusion initiatives being slashed across the federal government. "The Pentagon's focus will be on warfighting."

Next week, senators will be facing Trump's other outside Cabinet choices, including particularly Kash Patel, a Trump ally who has published an enemies list, as the FBI director; Tulsi Gabbard as director of the Office of National Intelligence; and Robert F. Kennedy, Jr, the anti-vaccine advocate at Health and Human Services.

Hegseth himself was working the phones late on Friday to shore up his support, his confirmation at stake.

"He's a good man," Trump said of Hegseth while departing the White House to visit disaster-hit North Carolina and Los Angeles. "I hope he makes it."