image source, Reutersimage captionGeneral Mark Milley said the calls were made as part of his duty as Joint Chief of Staffs
Top US General Mark Milley has defended himself after a book reported he had "secret" phone calls with China amid concerns about then-President Donald Trump.
The calls last October and January were to reassure the Chinese military, Gen Milley said on Wednesday.
Mr Trump said the claims were fabricated and Republicans have called for the general to be fired.
President Joe Biden said he has "great confidence" in Gen Milley.
Gen Milley's spokesman said that the calls were in keeping with his "duties and responsibilities conveying reassurance in order to maintain strategic stability".
The phone calls to Chinese General Li Zuocheng were revealed on Tuesday in extracts from a new book by Washington Post investigative reporters. They were made just after the presidential election and after Mr Trump refused to accept his defeat.
The book, "Peril", said that after the January 6 riots, Gen Milley, Chairman of the Joint Chiefs of Staff, "was certain that Trump had gone into a serious mental decline in the aftermath of the election".
He was allegedly worried that Mr Trump could "go rogue", the book claims.
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He allegedly told the Chinese general that the "American government is stable" and reassured Gen Li that the US would not attack. If they did so, the Chinese would be warned first, the extract quotes him as saying.
The book also said that Gen Milley had told his staff that if Mr Trump ordered a nuclear strike, then he would have to confirm it before it was carried out.
Mr Trump accused Gen Milley of "treason" and described the claims as "fake news" in a statement.
Senior Republican Senator Marco Rubio has also called for Mr Biden to fire the general.
On Wednesday, White House press secretary Jen Psaki said: "The president has complete confidence in his leadership, his patriotism and his fidelity to our Constitution."
She added that Mr Biden has complete confidence in Gen Milley continuing to serve in his role.