Washington DC - President Donald Trump's former national security advisor Mike Waltz on Tuesday defiantly defended his use of a group chat to discuss military plans as he faced accusations of lying during a hearing to be US ambassador to the United Nations.

The editor-in-chief of The Atlantic magazine said in March that Waltz had mistakenly added him to a chat among top US officials on commercial messaging app Signal about the imminent US bombing of Yemen.

Senator Cory Booker of the rival Democratic Party accused Waltz of deliberately maligning the journalist by falsely saying that he infiltrated the group.

"I've seen you not only fail to stand up, but lie," Booker told Waltz.

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"I have nothing but deep disappointment in what I consider a failure of leadership on your part," Booker told Waltz.

Waltz pointed to guidance under former President Joe Biden that allowed the use of Signal, which is encrypted, and said the White House has not taken disciplinary action.

"The use of Signal was not only authorized, it's still authorized and highly recommended," Waltz said, while insisting the chat did not exchange "classified" information.

Senator Chris Coons, another Democrat, was incredulous over his explanation and voiced alarm that the White House has not taken any corrective action.

"You were sharing details about an upcoming airstrike – the time of launch and the potential targets. I mean, this was demonstrably sensitive information."

Waltz, a former congressman and special forces officer, survived little more than three months as national security advisor before Trump, on May 1, replaced him with Secretary of State Marco Rubio, who is juggling both jobs.

Waltz did not deny he has kept taking his salary, saying he was not "fired" and still served as "an advisor."

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Senator Jacky Rosen, raising the salary issue, contrasted Waltz's actions with his vow to "root out waste and unnecessary overhead at the UN."

Trump has aggressively cut US assistance overseas and pulled the US out of several UN-backed bodies.

Waltz vowed to press for reforms at the United Nations, accusing it of "antisemitism" and "radical politicization" for criticisms of Israel and the US, even though the US is the organization's largest funder.

The United Nations, he said, has "drifted from its core mission of peacemaking."

"The UN's overall revenue has quadrupled in the last 20 years, yet I would argue we have not seen a quadrupling of world peace," Waltz said.