Donald Trump has tweeted a Game of Thrones-themed poster just moments after the completion of a press conference by the attorney general derided by critics as an attempt to protect the president ahead of the Mueller report's release.

"No collusion, no obstruction. For the haters and the radical left Democrats - GAME OVER," reads the message, which is written in font similar to that used on the popular HBO fantasy drama.

The poster, which features an image of Mr Trump turned away and surrounded by mist, appears an attempt by the president to pre-empt the upcoming release of Robert Mueller's report on the Trump campaign's links to Russia, and whether the president obstructed justice.

William Barr, the attorney general, had just seconds earlier finished taking questions from reporters following a speech in which he repeatedly parroted Mr Trump's line that the special counsel probe had found "no collusion".

Eager to get in the last word ahead of the public release of the special counsel's report, Mr Barr laid out in advance what he said was the "bottom line".

While Mr Mueller drew no conclusion about whether MrTrump had obstructed justice in the investigation, Mr Barr said he and deputy attorney general Rod Rosenstein personally had concluded that while Mr Trump was "frustrated and angry" about the Mueller probe, nothing the president did rose to the level of an "obstruction-of-justice offence".

Mr Barr said Mr Mueller's report examined 10 episodes pertaining to Mr Trump and obstruction. He said the president did not exert executive privilege to withhold anything in the report, and he said the president's personal lawyer had requested and gotten a chance to review the report before its public release.

The Justice Department will release a redacted version of the special counsel's report later, opening up months, if not years, of fights over what the document means in a deeply divided country.

Earlier, Mr Trump used a tweet to claim the investigation into Russian election meddling was "The Greatest Political Hoax of all time!".

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Also minutes after the press conference ended, House judiciary chairman Jerry Nadler released a three-page letter asking Mr Mueller to testify soon before his panel about the 400-page document.

The New York Democrat tweeted that Congress and the public need to hear directly from Mr Mueller to "better understand his findings". He wrote in his letter he wants Mr Mueller to testify by 23 May and asked for his "prompt attention" to the request.

Mr Barr said at his news conference he did not object to Mr Mueller testifying.