It's no secret Donald Trump isn't a fan of the media.

He once again made that clear during an aggressive press conference in which he defended his administration as a "fine-tuned machine" and denied his team had made inappropriate contact with Russian officials during the election campaign.

But this morning, the US President dramatically stepped up his attack on news outlets on his preferred medium Twitter:

The FAKE NEWS media (failing @nytimes, @NBCNews, @ABC, @CBS, @CNN) is not my enemy, it is the enemy of the American People!

It's not the first time that Mr Trump has referred to unfavourable coverage as "fake news", but for many journalists, the claim that they were "the enemy of the American people" crossed a line.

David Frum, a former speechwriter for George W Bush and a senior editor at The Atlantic, told MSNBC that the comment looked "a lot like an incitement to vigilante violence".

"I think we are seeing here just a violation of every decent democratic, constitutional norm we ever thought the leaders of the country should be guided by," he said.

Other journalists suggested Mr Trump's language was of the kind you'd expect to hear in a repressive state:

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Even before Mr Trump's latest comments, Rice University historian Douglas Brinkley said Mr Trump was unique among US presidents with his attacks on the press.

"There has never been a kind of holistic jihad against the news media like Trump is executing," he said.

"Trump is determined to beat and bloody the press whenever he finds himself in a hole."

Princeton historian Julian Zelizer agreed:

"The scale and scope of this is unlike anything that we've seen in the past."

The closest parallel might be with Richard Nixon, who told the chairman of the Joint Chiefs of Staff in a taped conversation that "the press is your enemy".

There have been other notable run-ins between presidents and the media.

In 1807, President Thomas Jefferson wrote to an editor in terms that Mr Trump might approve:

"Nothing can now be believed which is seen in a newspaper. Truth itself becomes suspicious by being put into that polluted vehicle."

But that was a far cry from what he wrote in 1787 before he became president:

"Were it left to me to decide whether we should have a government without newspapers, or newspapers without a government, I should not hesitate a moment to prefer the latter."

More recently, during his presidential campaign in 2000, George W Bush was overheard describing a New York Times reporter as a "major league asshole".

ABC/AP