US defence officials said James Anderson, the top policy adviser at the Pentagon, submitted his resignation Tuesday, a day after President Donald Trump fired Defense Secretary Mark Esper. Anderson has been the acting undersecretary for policy since June. Previously he served as the deputy undersecretary since his confirmation for that job in August 2018.

Trump's firing of Esper comes as he has refused to concede his election loss to Democrat Joe Biden.

Defense officials spoke about Anderson's resignation on condition of anonymity to discuss personnel matters.

A wide range of policy staff positions in the Pentagon have been filled with people on an acting basis over the past year or more, as a number of staff have left or have not been confirmed.

Chris Miller, who was tapped to serve as the Pentagon chief on Monday after Esper was fired, was in his second day in the building, meeting with top staff.