Trump Image Credit: Bloomberg

Washington: President Donald Trump's appointment of Matthew Whitaker to lead the US Department of Justice was called "unlawful" on Tuesday in a court challenge less than one week after he was named acting attorney general.

The state of Maryland asked a federal judge to block Whitaker from acting on behalf of the department in an ongoing court case on healthcare policy, arguing that he had not been approved by the US Senate as required by law.

But Maryland state Attorney General Brian Frosh made clear that the challenge was also a political move against someone that Democrats believe Trump chose to protect his presidency from the Russia collusion investigation of Special Counsel Robert Mueller.

"President Trump's brazen attempt to flout the law and constitution in bypassing Deputy US Attorney General Rosenstein in favor of a partisan and unqualified staffer cannot stand," Frosh said in a statement.

Trump, he said, has chosen "an unqualified and unconfirmed partisan to be the nation's chief law enforcement officer in order to protect himself rather than the rule of law".

Frosh's request for an injunction against Whitaker was the first official challenge to his appointment last Wednesday to replace fired attorney general Jeff Sessions.

The appointment has raised controversy because of Whitaker's limited experience and his record of vocal opposition to the Mueller investigation, which he has now taken control of from Rosenstein.

Maryland said the appointment violated a fundamental law requiring the top government officials to be confirmed by the Senate - which Rosenstein has been.

Legal experts have warned that Whitaker could jeopardize Justice Department legal cases if his appointment is found to be illegal.

"Whitaker's appointment is unlawful," the state said in a court filing, saying it could damage Maryland's ongoing healthcare case against the government.

Whitaker had been Sessions' chief of staff for a year, acting a liaison to the White House as Trump's relations with Sessions became increasingly strained.

Before that, he was a federal prosecutor in Iowa who had lost two races for political office and then a television and radio commentator.

In that role last year he made repeated comments dismissing the basis for the Mueller investigation and suggesting it be starved for funds to shut it down.