President Trump on Wednesday lashed out over a critical news report and escalated his previous attacks on the media by suggesting that news organizations he disagrees with be shut down, alarming free-speech advocates who compared the tactics to intimidation efforts by the Nixon administration.

The president's outbursts, which marred an Oval Office meeting with Canadian Prime Minister Justin Trudeau, came in reaction to an NBC News report that he had pushed senior aides in July for a major expansion of the U.S. nuclear arsenal. The president's request reportedly prompted Secretary of State Rex Tillerson to refer to Trump as a "moron" after the meeting at the Pentagon.

On Twitter, Trump called the report "pure fiction made up to demean" him and questioned whether networks that report "Fake News" should be stripped of their broadcasting licenses — although the Federal Communications Commission licenses individual stations and affiliates, not networks.

"Bad for the country!" Trump wrote.

Later, in response to questions from reporters in the Oval Office, Trump denied expressing a desire for more nuclear weapons and again criticized NBC.

"It's frankly disgusting the way the press is able to write whatever they want to write," Trump said. "And people should look into it."

Trump said he was not calling for limits to be imposed on the media, but he said that "the press should speak more honestly."

"I mean, I've seen tremendously dishonest press," he said. "It's not even a question of distortion. . . . And then they have their sources that don't exist."

But Wednesday night, Trump reiterated his call for possibly challenging networks' licenses over their news coverage.

"Network news has become so partisan, distorted and fake that licenses must be challenged and, if appropriate, revoked. Not fair to public!" he said in a message on Twitter.

Trump's diatribe marked the latest attack against the news media by a president who, according to people close to him, has felt increasingly frustrated over his stalled legislative agenda and political troubles. While Trump has long attacked news coverage of his administration as unfair, his latest missives have morphed into vague threats of government action at his perceived adversaries.

1 of 83 Full Screen Autoplay Close Skip Ad × Scenes from Trump’s second six months in office View Photos A look at the second half of the president’s first year in the White House. Caption A look at the second half of the president’s first year in the White House. Jonathan Ernst Buy Photo Wait 1 second to continue.

Last week, angered by the ongoing investigations into his campaign's ties to Russia, Trump suggested that the Senate Intelligence Committee investigate news outlets over "fake news." Over the weekend, he expressed disdain at late-night television hosts over their "anti-Trump" material and proposed bringing back the Fairness Doctrine, a rule phased out in 1987 that had required broadcasters to provide "equal time" for divergent political views on certain issues.

First Amendment advocates roundly condemned the president over his remarks, calling them an assault on the Constitution.

"Mr. President: Words spoken by the President of the United States matter. Are you tonight recanting of the oath you took on January 20th to preserve, protect, and defend the First Amendment?" Sen. Ben Sasse (R-Neb.) said in a statement.

"I think it's dangerous and has a chilling effect," said Michael Copps, who served as chairman of the FCC in 2009 under President Barack Obama. "I worry not just about somebody like NBC, which has a lot of resources to defend themselves, but especially about small, independent broadcasters who express opinions but do not have resources to do battle with the president."

Legal experts called the president's threat against NBC empty, noting that the FCC does not grant licenses to networks. Furthermore, they said, it is rare for individual stations' licenses to be stripped over political concerns or for other reasons.

"Not how it works," Jessica Rosenworcel, a member of the FCC, wrote on Twitter in response to Trump, linking to an FCC report on how stations are regulated.

"Obviously, when a public official, much less the president, threatens media outlets with any kind of legal proceedings, it is a cause for grave concern as a First Amendment matter," said Andrew Schwartzman, a media law specialist at the Georgetown University Law Center. "But as a practical matter there is no possible legal jeopardy for Comcast television licenses." Comcast, the cable giant, owns NBC.

Schwartzman noted the historical precedent, when allies to President Richard M. Nixon challenged individual licenses of television stations owned by The Washington Post Co. in 1973 during The Post's ongoing investigation into the Watergate scandal.

Those challenges were baseless and were unsuccessful, Schwartzman said.

"The Post was never under serious legal threat," he said, adding that Trump "is ignorant over details like what the law actually requires or permits."

NBC reported that Trump's reaction over the nuclear stockpile came after senior advisers showed him information charting its steady decline in numbers since the 1960s during the meeting at the Pentagon in July. Trump then expressed a desire to expand the arsenal by up to 10 times its current size, according to the report.

Trump is preparing for an 11-day trip next month to five Asian nations, during which he will seek to bolster international support to pressure North Korea to curb its nuclear weapons and ballistic-missile testing. During a speech at the United Nations last month, Trump said the United States was prepared to "totally destroy" the North if necessary, and he derisively referred to dictator Kim Jong Un as the "Rocket Man." North Korea responded that Trump's threats amounted to a declaration of war.

Tillerson said during a visit to Beijing two weeks ago that the administration was "probing" for channels of direct communication with Pyongyang to ratchet down tensions. But Trump quickly undercut the nation's top diplomat, saying on Twitter that it was a waste of time.

"Recent reports that the President called for an increase in the U.S. nuclear arsenal are absolutely false," Defense Secretary Jim Mattis said in a statement. "This kind of erroneous reporting is irresponsible."

In the Oval Office, Trump said of the nuclear arsenal: "We won't need an increase, but I want modernization and I want total rehabilitation. It's got to be in tiptop shape." He said a tenfold increase in the stockpile is "totally unnecessary, believe me. Because I know what we have right now."

NBC did not formally respond to Trump's attacks on the network. Courtney Kube, a national security reporter for the network, wrote on Twitter: "NBC News didn't report Trump 'called for' more nucs. On the contrary, we reported Trump said he wanted more but no one took it as an order."

Last week, after Trump suggested the Senate committee investigate news organizations, White House press secretary Sarah Huckabee Sanders defended the president, calling him "an incredible advocate of the First Amendment."

Reporters, she said, "have a responsibility to tell the truth, to be accurate," and she cited a study from Pew Research Center that found 5 percent of news stories about Trump in his first 60 days in office were positive.

Sanders accused reporters of being consumed by "petty palace intrigue" — stories about personnel moves and internal West Wing squabbles — instead of focusing on improvements in the economy and the administration's campaign against terrorist groups.

Trump, for his part, has been consumed by the negative coverage and has railed against CNN, the New York Times, The Washington Post and other outlets for months. In July, he tweeted a doctored video clip of him in a WWE wrestling match from years ago tackling a figure with a CNN logo superimposed on the head.

In recent weeks, Trump has attacked ESPN over criticism of him by an anchor — suggesting the network apologize to him and blaming her in a tweet Tuesday for contributing to the sports network's declining ratings and subscription base.

Last month, when the anchor, Jemele Hill, referred to Trump as a "white supremacist," Sanders called it a "fireable offense."

Ben Wizner, director of the American Civil Liberties Union's Speech, Privacy, and Technology Project, said Trump's attacks risk "outrage fatigue." But, he added, "a call to challenge the licenses of news organizations for doing their jobs is genuinely shocking. This is the language of an autocrat, not the elected leader of a democracy."