U.S. President Donald Trump announced on Saturday that he would nominate Army General Mark Milley to be the next chairman of the Joint Chiefs of Staff.

Trump said on Twitter that Milley, currently the Army chief of staff, would take over as the top U.S. military commander from Marine General Joseph Dunford at a date to be determined.

"I am thankful to both these incredible men for their service to our country," Trump said on Twitter.

Later on Saturday, departing the White House for Philadelphia for the annual Army-Navy football game, Trump told reporters that Milley is "a great gentleman, he's a great patriot, he's a great soldier."

Trump's announcement comes months earlier than expected. Dunford would normally remain as chairman of the Joint Chiefs of Staff until a scheduled October 1, 2019, handover.

Speaking to that point, Trump said without elaborating, "They usually give quite a bit of notice for a lot of reasons."

A spokesman for the Joint Chiefs of Staff said that indications were that Dunford would complete his term, but he referred questions on the matter to the White House.

"General Dunford congratulates General Milley on his selection as the nominee to be the next chairman of the Joint Chiefs of Staff. He has served with General Milley in peacetime and combat and has the highest regard for his leadership," said Colonel Patrick Ryder.

A spokesman for Defense Secretary Jim Mattis, Lieutenant Colonel Mike Andrews, said, "We are aware of the president's nomination and share his confidence" in Milley.

Lindsey Graham, a senior Republican member of the Senate Armed Services Committee, called Milley "a great choice" to succeed Dunford.

Milley is "a battle-tested commander and Pentagon reformer who will be a worthy successor to General Dunford," Graham said in a statement. He called Dunford "one of the most dedicated officers I've ever known."