CNN and MS NOW journalists have been denied access to the White House after US President Donald Trump announced he was banning the outlets.

An MS NOW reporter and a photographer tried entering the White House grounds on Saturday morning, but their badges would not scan and an officer confiscated them, the outlet reported.

CNN also said Saturday morning that one of its reporters was denied access and had her badge seized.

Trump said on Friday he was barring MS NOW, CNN, and Politico - three major US media outlets - from the White House, accusing them of writing "fiction or lies" about his administration. Press freedom advocates called the move "unconstitutional".

"The officer asked me to hand over my badge. He said that it was disabled," MS NOW reporter Akayla Gardner said. "I asked why I was not able to get inside. He said it was above him."

Gardner added that while her and her colleague's badges were disabled, an MS NOW producer was still able to scan her badge to get in, though she was not given an explanation as to why the producer's badge still worked.

CNN reporter Betsy Klein said a Secret Service agent told her her badge had been deactivated.

An MS NOW spokesperson said in a statement that the White House "belongs to the American people and the decisions made inside are funded by our tax dollars".

"MS NOW intends to take any and all steps necessary to defend our First Amendment rights and the essential role of independent journalism in our democracy."

It was not clear whether Politico reporter were at the White House on Saturday. Following Trump's ban, all three outlets have spoken out against the decision, accusing Trump of violating their constitutional rights to free speech and freedom of the press.

"We have a right under the US Constitution to do our reporting without hindrance or interference from the government and this ban is an illegal assault on this fundamental right," CNN said.

In his announcement, Trump did not mention specific stories or reporting from the three outlets that prompted the move, but later said "it's really just cumulative stories over the last few years".

Trump added that there "may be others to join them", but has not given a directive banning additional news outlets. When later asked by a reporter what he meant by others to come, Trump responded: "Others to come in terms of fake news? Well, you know, the New York Times is fake news, the Washington Post is fake news."

Several groups dedicated to the freedom of the press have called out Trump's White House ban as illegal and unconstitutional.

A major group that provides legal services to the media - the Reporters Committee for Freedom of the Press - said it expected Trump's ban to be struck down quickly by the courts because it was "flatly unconstitutional".

"The First Amendment is clear that once the White House invites in some journalists, it can't ban others because it doesn't like their reporting," said Bruce Brown, president of the Reporters Committee for Freedom of the Press, which provides legal services to the media.

It's the latest in a series of moves by the administration to target media outlets Trump says are critical of him, including legal action against US and international outlets like The New York Times, The Wall Street Journal, and the BBC.

Some of the lawsuits have resulted in media organisations agreeing to pay multimillion dollar settlements. Trump's lawsuit against the BBC is ongoing.