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