In comments made last week on Truth Social, the former president said the top general should face the death penalty for a conversation he had with his Chinese counterpart.
He claimed Mr Milley went behind his back to communicate with China during the final months of the Trump administration.
“This guy turned out to be a Woke train wreck who, if the Fake News reporting is correct, was actually dealing with China to give them a heads up on the thinking of the President of the United States,” Mr Trump wrote. “This is an act so egregious that, in times gone by, the punishment would have been DEATH! A war between China and the United States could have been the result of this treasonous act.”
His comments prompted former House speaker Nancy Pelosi to say Mr Trump should be barred from public office.
“To engage a language like that… it should be something that would eliminate someone as a prospect for any public office,” she said.
Mr Milley has denied that there was anything treasonous about the calls he made to China, and he told CBS he is taking “appropriate measures” to ensure his safety following Mr Trump’s comments.
“I’ll take appropriate measures to ensure my safety and the safety of my family,” Mr Milley said.
The outgoing top general also said that he has been “faithful to the Constitution of the United States” for nearly 45 years, adding he and his family “sacrificed greatly for this country.”
Mr Milley said he won’t directly comment on the specifics of Mr Trump’s post, but he insisted: “This military, this soldier, me, will never turn our back on that Constitution.”
It comes after a 2021 book by Bob Woodward and Robert Costa revealed Mr Milley assured General Li Zuocheng of the People’s Liberation Army that the US would not launch an attack against China toward the end of the Trump administration.
Mr Milley’s spokesperson said the general’s calls to China were part of his regular communications with defence chiefs worldwide. The spokesperson described the calls as being crucial to reducing tensions between nations, as well as “avoiding unintended consequences or conflict.”
“His calls with the Chinese and others in October and January were in keeping with these duties and responsibilities conveying reassurance in order to maintain strategic stability,” his spokesperson said. “All calls from the chairman to his counterparts, including those reported, are staffed, coordinated and communicated with the Department of Defense and the interagency.”