US broadcaster Fox News is backing arch-rival CNN in its lawsuit against the White House for revoking the press credentials of one of its Washington correspondents.

Key points: White House argues it was lawful to punish Acosta for his behaviour during press conference

White House argues it was lawful to punish Acosta for his behaviour during press conference Fox News president said passes for working White House journalists "should never be weaponised"

Fox News president said passes for working White House journalists "should never be weaponised" Fox joins group of 12 news organisations supporting CNN

CNN launched legal action against the Trump administration after reporter Jim Acosta had his "hard pass" — which gives him daily access to White House grounds — revoked after a heated exchange with the President during a press conference after last week's midterm elections.

Mr Trump's administration is arguing it "was lawful" to punish Acosta for his behaviour during the conference, and rejected the idea he was "otherwise eligible" for White House access.

"The President and his designees in the White House Press Office have exercised their discretion not to engage with him and, by extension, to no longer grant him on-demand access to the White House complex so that he can attempt to interact with the President or White House officials," it said.

Mr Trump himself, in an interview with The Daily Caller, was uncertain how the court fight would end, saying: "We'll see how the court rules. Is it freedom of the press when somebody comes in and starts screaming questions and won't sit down?"

Mr Trump said, "guys like Acosta" were "bad for the country … he's just an average guy who's a grandstander who's got the guts to stand up and shout".

But Fox News president Jay Wallace said passes for working White House journalists "should never be weaponised", and Fox said it would file an amicus brief — a legal brief filed by non-litigants who have a strong interest in the subject matter — in support of CNN.

"Fox News supports CNN in its legal effort to regain its White House reporter's press credential," Mr Wallace said in a statement.

"While we don't condone the growing antagonistic tone by both the President and the press at recent media avails, we do support a free press, access and open exchanges for the American people."

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Fox joined a group of 12 news organisations, including NBC, The New York Times, The Washington Post and the Associated Press, who planned to file amicus briefs.

"Whether the news of the day concerns national security, the economy, or the environment, reporters covering the White House must remain free to ask questions. It is imperative that independent journalists have access to the President and his activities, and that journalists are not barred for arbitrary reasons," the law firm Ballard Spahr said in a statement on behalf of the news organisations.

A District Court judge said he would announce on Thursday (local time) whether to grant a restraining order to force the White House to return the press pass.

CNN warns of 'chilling effect' over ban

The White House's explanations for why it seized Acosta's hard pass have shifted over the past week. Acosta has repeatedly clashed with Mr Trump and press secretary Sarah Huckabee Sanders in briefings over the past two years.

Mr Trump erupted into anger last week during a news conference when Acosta questioned him about the so-called migrant caravan travelling through Mexico and about an ongoing investigation into Russian meddling in the 2016 presidential election.

"That's enough, that's enough," Mr Trump said as a White House intern attempted to take the microphone from Acosta.

"You are a rude, terrible person," the President added.

The White House suspended his credentials later that day, with press secretary Sarah Sanders accusing Acosta of putting his hands on the intern who was trying to take the microphone from him.

Ms Sanders called his behaviour "absolutely unacceptable", and later tweeted a video of the incident that was said to be "doctored".

Sorry, this video has expired A frame-by-frame analysis between the original C-span video (left) and a video shared by Sarah Sanders (right).

Video of the encounter showed Acosta pulling back as the intern moved to take the microphone. Acosta appeared to brush her arm as he tried to hold onto the microphone.

"Pardon me, ma'am," he can be heard saying.

He called the White House accusations a lie.

"While the suit is specific to CNN and Acosta, this could have happened to anyone," CNN said.

"If left unchallenged, the actions of the White House would create a dangerous chilling effect for any journalist who covers our elected officials."

ABC/wires