Defense Secretary Lloyd Austin and Joint Chiefs of Staff Chairman Army General Mark Milley hold a news conference

Joint Chiefs of Staff Chairman Army Gen. Mark Milley holds a news briefing at Pentagon in Arlington, Va., Wednesday, Aug. 18, 2021.

President Joe Biden has "complete confidence" in Gen. Mark A. Milley, the chairman of the Joint Chiefs of Staff, a White House spokeswoman said Wednesday, following a report that the country's top military officer privately conferred with his Chinese military counterpart to avert armed conflict with the United States late in the Trump administration.

"I can assure you all ... that the president knows General Milley," White House spokeswoman Jen Psaki said. "He has been chairman of the Joint Chiefs for almost eight months of his presidency. They've worked side by side through a range of international events. And the president has complete confidence in his leadership, his patriotism and his fidelity to our Constitution."

Psaki's comments came as Col. Dave Butler, a spokesman for Milley, said that Milley had acted constitutionally and within his established responsibilities.

In a statement, Butler said 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."

Secret calls from Milley to Gen. Li Zuocheng of the People's Liberation Army in October and January are detailed a book by Washington Post associate editor Bob Woodward and national political reporter Robert Costa.

Butler's statement largely confirmed the reporting in the book.

The defense of Milley's conduct came a day after former president Donald Trump, speaking on the conservative television network Newsmax, labeled the chairman's reported actions "treason" and said, "I did not ever think of attacking China."

According to the book, one of Milley's calls took place on Oct. 30, 2020, four days before the election that unseated Trump, and the other on Jan. 8, 2021, two days after the Capitol siege carried out by Trump supporters in a quest to overturn the vote.

The first call was prompted by Milley's review of intelligence suggesting the Chinese believed the United States was preparing to attack. That belief, the authors write, was based on tensions over military exercises in the South China Sea, and deepened by Trump's belligerent rhetoric toward China.

In the second call, placed to address Chinese fears about the events of Jan. 6, Li wasn't as easily assuaged, even after Milley promised him, "We are 100 percent steady. Everything's fine. But democracy can be sloppy sometimes," the authors write.

In his statement Wednesday, Butler said Milley "regularly communicates with Chiefs of Defense across the world, including with China and Russia."

"These conversations remain vital to improving mutual understanding of U.S. national security interests, reducing tensions, providing clarity and avoiding unintended consequences or conflict," Butler said. "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."

News of Milley's conversations prompted harsh criticism from Republicans on Capitol Hill and prominent military observers, many of whom called for his resignation.

"You must immediately dismiss General Milley," Sen. Marco Rubio, R-Fla., the top-ranking Republican on the Senate Intelligence Committee and a member of the Foreign Relations panel, wrote in a letter to Biden on Tuesday. "America's national security and ability to lead in the world are at stake.

Rubio said Milley "worked to actively undermine the sitting Commander in Chief" - then Trump - "and contemplated a treasonous leak of classified information to the Chinese Communist Party."

His alleged breach of the chain of command - as chairman of the Joint Chiefs of Staff, Milley's role is advisory - also rankled at least one high-profile Trump critic.

Former Lt. Col. Alexander Vindman, a star witness in the first impeachment proceedings against Trump, went on Twitter on Tuesday to declare that if the allegations in the book were true, "Milley must resign."

"He usurped civilian authority, broke Chain of Command, and violated the sacrosanct principle of civilian control over the military," Vindman wrote. "It's an extremely dangerous precedent. You can't simply walk away from that."

While Republicans on Capitol Hill have echoed Rubio's contention that Milley's reported actions constitute a fireable offense, Democrats have maintained that his statements and actions were justified.

"It is breathtaking to think of the lengths that Milley and others went to to avert the disasters Trump was creating at the end of his presidency," Sen. Richard J. Durbin, D-Ill., told reporters, adding that he was "not at all" concerned if Milley had overstepped his authority.

"It is a shame we reached that point in America's history that's necessary, and I think he did the responsible thing to keep America out of war," Durbin said.

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The Washington Post's Isaac Stanley-Becker contributed to this report.