Washington (CNN) President Donald Trump on Saturday nominated Army Chief of Staff Gen. Mark Milley to be the next chairman of the Joint Chiefs of Staff.

"I am pleased to announce my nomination of four-star General Mark Milley, Chief of Staff of the United States Army -- as the Chairman of the Joint Chiefs of Staff, replacing General Joe Dunford, who will be retiring," Trump posted on Twitter Saturday morning. "I am thankful to both of these incredible men for their service to our Country!"

Trump said the date of transition is still to be determined.

Marine Corps Gen. Joseph Dunford, who took on the role in 2015, is expected to serve out his term, which expires later next year.

The chairman is the highest-ranking military officer in the country and serves as the principal military adviser to the President. It is a role that requires Senate confirmation.

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