Trump hinted earlier on Friday that he would be announcing new military leadership on Saturday when he attends the Army-Navy football game.
Milley would succeed Marine General Joseph Dunford, who would normally stay in office as chairman until a slated Oct. 1, 2019 handover date. It was unclear whether that date would be moved earlier as a result of an early announcement about his successor, if confirmed.
One U.S. official, speaking to Reuters on condition of anonymity because the matter was not yet public, suggested that keeping Dunford in the job until Oct. 1 would be difficult for both him and Milley in such a scenario.
The Pentagon declined comment and was not expected to issue any statements before Trump speaks on Saturday.
Milley, the Army's chief of staff, and Air Force Chief of Staff General David Goldfein have been seen as the two leading contenders to replace Dunford next year.
The change is part of a routine periodic rotation of top military posts that will also soon include new heads of military services, including the Navy and Army.
Milley, a respected Pentagon leader who served in Iraq and Afghanistan, is not expected to usher in any major strategic shifts. The chairman of the Joint Chiefs of Staff, the top U.S. military officer, is not a political job.
One of Milley's major innovation at the Army was the creation this year of special brigades to help advise local forces in wars like the one in Afghanistan. The goal was to allow other forces to increasingly focus on higher-end military challenges from China and Russia. (Reporting by Phil Stewart and Steve Holland Editing by Susan Thomas and Jeffrey Benkoe)
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