US President Donald Trump called Friday 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."

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 on Thursday.

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

Presidents have historically bent over backward -- at least in public -- 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 -- a

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