The Pentagon confirmed Wednesday that Gen. Milley made two calls to his counterpart in China in the final months of the Trump administration.
A Joint Staff spokesperson said the calls were aimed at maintaining stability.
The White House said that it has complete confidence in Milley, whom critics are accusing of "treasonous" behavior.
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The top US general called his counterpart in China twice in the final months of the Trump administration to reassure the Chinese and avoid conflict, the Pentagon said Wednesday, characterizing the interactions as a vital part of his duties but not addressing specific quotes attributed to him in recent reporting.
Washington Post reporters Bob Woodward and Robert Costa reported in the new book "Peril" that Chairman of the Joint Chiefs of Staff Gen. Mark Milley made two secret phone calls to his counterpart in China's People's Liberation Army, Gen. Li Zuocheng, in part because there were concerns that President Donald Trump had declined mentally and might spark a war.
One call was just days before the presidential election, and the other was two days after the Capitol riot.
Milley reportedly told Li during their first phone call that he wanted to assure him "that the American government is stable and everything is going to be okay." Milley told the Chinese officer that "we are not going to attack or conduct any kinetic operations against you."
The book also said that the general pledged to give the Chinese advanced warning of an attack.
During the second call, Milley told Li that "we are 100 percent steady. Everything's fine. But democracy can be sloppy sometimes." The call reportedly did not alleviate Li's discomfort.
Following reports on Milley's calls to his Chinese counterpart, the general has faced criticism from Trump, who appears to have been unaware of the calls, as well as Republican lawmakers, who have said that if Milley acted as described, then his actions were "treasonous." The general has faced several calls to resign.
A defense official told Politico that reporting on Milley's calls had been "grossly mischaracterized" and that the calls were neither frantic or unusual. One official who was present said Milley received permission from acting Secretary of Defense Chris Miller before calling.
Pentagon Press Secretary John Kirby said Wednesday that it is "not only common, it's expected that a chairman of the Joint Chiefs of Staff would continue to have counterpart conversations," adding that "frequent communication with countries like Russia and China is not atypical."
"A part of the value of having these conversations, particularly with countries like Russia and China, with which we are experiencing tension, is to try to reduce the risk of miscalculation and conflict, to try to take down tensions, to make clear what our national security interests are," he said.
The Office of the Chairman of the Joint Chiefs of Staff made a similar point in a separate statement on the calls.
"The Chairman of the Joint Chiefs regularly communicates with Chiefs of Defense across the world, including with China and Russia," Col. Dave Butler, a spokesperson for the Joint Staff, said in a statement. "These conversations remain vital to improving mutual understanding of US national security interests, reducing tensions, providing clarity and 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," the statement read. "All calls from the Chairman to his counterparts, including those reported, are staffed, coordinated and communicated with the Department of Defense and the interagency."
Fox News' Jennifer Griffin reported late Tuesday that there were at least 15 people on the video teleconference calls. In addition to the calls to China, Milley also reportedly made more than a dozen calls to NATO allies in the wake of the Jan. 6 riot.
"Milley continues to act and advise within his authority in the lawful tradition of civilian control of the military and his oath to the Constitution," Butler said in his statement.
White House press secretary Jen Psaki also defended Milley, saying on Wednesday that the reports on Milley are "anonymous unconfirmed reports about conversations with limited context." She added that the president has "complete confidence in his leadership, his patriotism and his fidelity to our Constitution."
Update: This post was updated with additional remarks from Pentagon spokesman John Kirby.