Trump calls for charges against opponents

US PRESIDENT Donald Trump has called for more of his political opponents to face criminal charges as he cheered the indictment of former FBI director James Comey and branded him a "dirty cop".

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FORMER FBI Director James Comey.

(AFP)

The Republican's comments step up an extraordinary, undisguised campaign of retribution against those who oppose him that defies decades of norms in US politics.

"Frankly I hope there are others," Trump told reporters at the White House when asked about Comey's indictment last week, describing his opponents as "corrupt, radical-left Democrats".

"It's not revenge. It's also about the fact that you can't let this go on."

Presidents have historically striven to show clear separation between the White House and the Justice Department. Trump has smashed that precedent, making clear he intends to influence Comey's case - and others.

Comey was charged with making false statements and obstruction of justice in connection with the probe he conducted into whether Trump colluded with Russia to win the 2016 election.

"He's a dirty cop, he's always been a dirty cop," Trump told reporters.