US President Donald Trump has escalated his efforts to discredit the Russia investigation, saying the FBI has been caught in a “major spy scandal”.

Trump claims the FBI used a secret informant to determine whether some of his campaign aides were working with Russia ahead of the 2016 election.

“SPYGATE could be one of the biggest political scandals in history!” Trump said in a Twitter tirade.

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The president and his Republican supporters in Congress are now demanding information on the outside informant, claiming it is proof that the Obama administration was trying to spy on his campaign for political reasons.

The White House has negotiated rare access to classified documents for Trump’s congressional allies in a briefing expected today.

“Look how things have turned around on the Criminal Deep State,” Trump tweeted.

“They go after Phony Collusion with Russia, a made up Scam, and end up getting caught in a major SPY scandal the likes of which this country may never have seen before! What goes around, comes around!”

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The president’s comments came a day after he increased pressure on the Justice Department, declining to say whether he has confidence in deputy attorney general Rod Rosenstein, and calling for an investigating into the handling of the Russia probe.