Washington | In another blow to Donald Trump's besieged administration, US Defence Secretary Jim Mattis resigned in protest over the president's controversial decision to withdraw troops from Syria and his contempt for America's closest allies.

Described as the last "adult" in Mr Trump's cabinet where he was seen as a stabilising force in an administration suffering rates of top-level staff turnover, General Mattis released a withering resignation letter that failed to say anything positive about the President.