'Hold fast'

Defense Secretary Jim Mattis bade farewell to the Pentagon on Monday, telling the US military to "hold fast" after he quit over a series of fundamental differences with President Donald Trump.

Mattis resigned December 20, after Trump stunned the US establishment by ordering a full troop withdrawal from Syria.

"Our department is proven to be at its best when the times are most difficult," Mattis said in a brief memo to the Pentagon, an apparent reference to the turmoil in Washington.

"So keep the faith in our country and hold fast, alongside our allies, aligned against our foes."

Mattis, a scholar who frequently backs his views with historical anecdotes, also quoted a telegram President Abraham Lincoln sent to General Ulysses Grant in February 1865, near the end of the American Civil War.

"Let nothing which is transpiring, change, hinder, or delay your military movements, or plans," the former Marine Corps general quoted Lincoln as saying.

He added that he was confident military members are "undistracted from our sworn mission to support and defend the Constitution while protecting our way of life."

Mattis was followed in his resignation by Brett McGurk, the US special representative to the coalition fighting the Islamic State movement, which Trump declared to be defeated.