News reported earlier in the day that "President Donald Trump said he wanted what amounted to a almost tenfold increase in the US nuclear arsenal during a gathering this past summer of the nation's highest-ranking national security leaders, according to three officials who were in the room".
Mattis, along with Secretary of State Rex Tillerson and other high-ranking administration officials, attended that meeting where the presentation reportedly happened.
After the meeting, NBC News reported, Tillerson was heard calling the president a "moron", a remark that the president has called "totally phony".
President Donald Trump boards Marine One on Friday at the White House.
"I never said that", he said during a news briefing with Canadian Prime Minister Justin Trudeau.
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Mr. Trump said that NBC News' reporting is "Pure fiction, made up to demean" and compared it to CNN, which he has previously also criticized and labeled as "fake news".
Trump lashed out at NBC News over a report indicating that he'd called for a drastic expansion of the US nuclear arsenal. "If you want me to weigh in against one entity, you can ask the whole Virginia delegation - we won't hardly speak to the Washington Post anymore".
Although US presidents have modernised weapon stockpiles over the years, any meaningful addition to the nuclear arsenal would violate treaty agreements. If it did what Trump is asking, it would pretty much be an American Ministry of Truth. "Bad for country!" Several hours later, he told pool reporters that "the way the press is able to write whatever they want to write" was "frankly disgusting".
"I don't utter [the term] 'fake news, '" Conway said, adding that her concern is instead with "incomplete coverage" of the president.
The threat provoked widespread anger on social media and elsewhere with many attacking the president for directly challenging the First Amendment, which guarantees the freedom of the press.