Trump hinted earlier on Friday that he would be announcing new military leadership on Saturday when he attends the Army-Navy football game.
"It will have to do with the Joint Chiefs of Staff and succession," he said.
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 brought forward as a result of an early announcement about his successor, if confirmed.
The Pentagon declined to comment and was not expected to issue any statements before Trump speaks on Saturday.
The expected change at the Pentagon comes as Trump overhauls his administration's top leadership positions in the wake of the November mid-term elections. On Friday, he announced his picks for attorney general and ambassador to the United Nations.
Milley will oversee a U.S. military already in transition, as it wraps up the fight against Islamic State in Syria, seeks to bomb the Taliban into submission in Afghanistan and preserve the balance in the Pacific as China's military flexes its muscles. It also faces an increasingly assertive Russia.
Like many of America's top generals, Milley comes from a military family, with both his parents having served in World War Two. Milley was two decades into his military carrier when al Qaeda militants launched the Sept. 11, 2001 attacks against Washington and New York City.
"When that happened, I said I can't retire: I had to stay until this thing is done," Milley told a publication linked to Princeton University, his alma mater, in 2014.
Milley served in Iraq and three tours in Afghanistan before becoming the Army's chief of staff in 2015.
As chairman of the Joint Chiefs of Staff, Milley would not be expected to usher in any major strategic shifts for the U.S. military. His views on the threats from Russia and China are in step with Pentagon top brass and the Trump administration.
He also shares U.S. military concerns about a war with North Korea, saying last year a full-blown conflict "would be horrific."
PRESSURE FROM CONGRESS
Milley would take over as the Pentagon's top military officer at a time when critics in Congress are accusing Trump of politicizing the military, including with his deployment of U.S. troops to the Mexico border.
When Democrats take over the majority in House of Representatives in January, they are expected to call military officers to testify about that and other Trump policies that have riled their constituents.
Still, the position of chairman is a non-political post. Milley would be entrusted with providing both Trump and Defense Secretary Jim Mattis with the best military advice on any national security issue, and carrying out orders.
As Army chief, Milley this year launched the Army's Futures Command, which looks at ways to usher in a new generation of advanced weaponry to preserve the United States' narrowing edge against potential adversaries like China and Russia.
"We are keenly aware that we need to shift gears rapidly into the modernization in order to make sure that we don't have parity" with Russia and China, he told the Senate in April.
Another of Milley's innovations at the Army was this year's creation of special brigades to help advise local forces in counter-insurgency wars, including the 17-year-old conflict in Afghanistan.
The goal of creating specialized brigades was to allow other forces to increasingly focus on the bigger military challenges posed by China and Russia.
Trump's announcement of Milley will put Dunford in an uncomfortable position.
Dunford was first selected by President Barack Obama in 2015 and then chosen by Trump to serve a second two-year term in 2017. Acting a bridge of sorts, Dunford has helped ensure continuity in military policy through two very different presidential administrations.
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. Still, there was no sign that Trump was seeking to push Dunford out early. (Reporting by Phil Stewart and Steve Holland; Additional reporting by Idrees Ali; Editing by Susan Thomas, Jeffrey Benkoe, Daniel Wallis and Richard Chang)
(This story has not been edited by Devdiscourse staff and is auto-generated from a syndicated feed.)