The Senate backed Esper, a former soldier and lobbyist for weapons maker Raytheon, to be President Donald Trump's second confirmed leader of the Pentagon by 90-8.

Esper, 55, received strong bipartisan support despite some sharp questioning during his confirmation hearing by Democratic Senator Elizabeth Warren about his ties to Raytheon and his refusal to extend an ethics commitment he signed in 2017 to avoid decisions involving the company.

Warren, a 2020 presidential hopeful, was the only member of the Senate Armed Services Committee to voice opposition to Esper's confirmation during the hearing.

Raytheon is the third-largest US defence contractor.

There has been no confirmed defense secretary since Jim Mattis resigned in December over policy differences with Trump.

Many members of congress from both parties had urged the Republican president to act urgently to fill the powerful position.

Senate Majority Leader Mitch McConnell called on members to support Esper as he opened the Senate on Tuesday morning.

"The nominee is beyond qualified. His record of public service is beyond impressive. His commitment to serving our service members if beyond obvious. And the need for a Senate-confirmed secretary of defence is beyond urgent," McConnell said.

Four of the eight "No" votes came from senators who are running for the 2020 Democratic presidential nomination - Cory Booker, Kirsten Gillibrand, Kamala Harris and Amy Klobuchar.

An Army veteran, Esper had served as a congressional aide and a Pentagon official under Republican president George W. Bush before working for Raytheon. He has been Army secretary since November 2017.

Trump's previous pick to be secretary of defence, former Boeing executive Patrick Shanahan, withdrew from consideration on June 18 after reports emerged of domestic violence in his family.

Australian Associated Press