Donald Trump has used his Twitter account to advance an unverified theory circulating among conservatives that the Justice Department planted a spy in his 2016 campaign and is now "out to frame him".

The president's early morning tweet seemed intended to undercut the Russia investigation by special counsel Robert Mueller, which he has repeatedly called a "witch hunt".

"Really bad stuff!" he tweeted.

Last week, the National Review raised the question of a possible FBI spy on Mr Trump's campaign.

The article cites work by congressman Devin Nunes, an ardent Trump supporter and head of the House intelligence committee, who has demanded information on an FBI source in the Russia investigation.

The New York Times reported separately this week that at least one government informant met Carter Page and George Papadopoulos, both former foreign policy advisers on Mr Trump's Republican campaign.

The newspaper attributed the information to current and former FBI officials.

In a tweet on Thursday, Mr Trump cited the National Review article suggesting the FBI source was really a "confidential informant in the campaign".

"If so, this is bigger than Watergate!" he tweeted.

He returned to the issue early on Friday, quoting Fox Business anchor David Asman: "Apparently the DOJ put a Spy in the Trump Campaign. This has never been done before and by any means necessary, they are out to frame Donald Trump for crimes he didn't commit."

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