US President Donald Trump in June nominated Mark Esper, previously the US secretary of the Army, to be the next defence secretary after his first choice, Acting Defence Secretary Patrick Shanahan, withdrew from consideration following accusations of domestic abuse.

There has been no confirmed US defence chief since James Mattis stepped down in December 2018, the longest period in the history of the Pentagon.

Esper was formally nominated to the position of defence secretary by the White House on Monday, a role he has been carrying out in an acting capacity since last month, Reuters reported. The White House said in a press release earlier that Esper's nomination has officially been sent to the Senate.

In a statement, cited by Reuters, the DoD said when Esper was nominated on Monday, he stopped serving as acting defence chief and reverted to his previous job: Army secretary.

The Trump administration has another new *acting* defense secretary, as of 3:04 p.m. pic.twitter.com/3DBn6E17SZ — Andrew deGrandpre (@adegrandpre) 15 July 2019

Until Esper is confirmed, Navy Secretary Richard Spencer will serve as acting defence secretary.

"My service as Acting Secretary of Defense affords the US Senate the opportunity to provide advice and consent on the President's nominee to serve as the next Secretary of Defense", Spencer said in a letter to Defence Department employees. "I expect to continue to serve in this role until a Secretary of Defense nominee is confirmed by the Senate and assumes office".

He added that he will resume his service as secretary of the Navy after a nominee is confirmed.

Esper, 55, served in the first Gulf War, worked as an aide on Capitol Hill, and spent several years as vice president for government relations at defence contractor Raytheon, according to Reuters.