President-elect Joe Biden nominated Kathleen Hicks as deputy defense secretary, making her the first woman to hold the number two position in the Pentagon if passed by the Senate.

"These respected, respected civilian leaders will help lead the Department of Defense with integrity and determination, protecting the lives and interests of the American people," Biden issued a statement on December 30.

He also nominated Colin Kahl as the deputy defense secretary for policy - the number three power official in the Pentagon.

Hicks served in the Department of Defense under Obama and is leading Biden's power transfer team in the Pentagon.

Earlier this month, Biden chose retired general Lloyd Austin, who oversees US forces in the Middle East under Obama, as defense secretary.

When Biden takes office, one of his toughest tasks will be to rebuild America's diplomatic and military alliances that have rifted under Trump's "America first" agenda over the past four years.