US President Donald Trump has asserted that FBI informants attempted to infiltrate his 2016 presidential campaign, saying “if so, this is bigger than Watergate!”

Mr Trump made the comment on the anniversary of Robert Mueller’s appointment as special counsel to head the Justice Department investigation into possible collusion between Russia and the Trump campaign.

It comes the same day Mr Mueller’s office was forced to give a federal court a classified memo describing the extent of his investigation.

Investigators are searching whether any of Mr Trump’s associates were coordinating with Russia to help the then-Republican candidate defeat his Democratic rival, Hillary Clinton.

Mr Trump, who has repeatedly maligned Mr Mueller’s investigation as a “witch hunt”, compared the Russian probe to the Watergate investigation more than four decades ago.

President Richard Nixon at the time ordered his attorney general and deputy attorney general to fire the special counsel investigating the 1972 break-in by five men at Democratic Party headquarters at the Watergate building in Washington.

Mr Nixon ultimately resigned from office as a result of the ensuing investigation.

“Wow, word seems to be coming out that the Obama FBI ‘SPIED ON THE TRUMP CAMPAIGN WITH AN EMBEDDED INFORMANT’,” Mr Trump tweeted.

Wow, word seems to be coming out that the Obama FBI “SPIED ON THE TRUMP CAMPAIGN WITH AN EMBEDDED INFORMANT.” Andrew McCarthy says, “There’s probably no doubt that they had at least one confidential informant in the campaign.” If so, this is bigger than Watergate!

— Donald J. Trump (@realDonaldTrump) May 17, 2018

“Andrew McCarthy says, ‘There’s probably no doubt that they had at least one confidential informant in the campaign.’ If so, this is bigger than Watergate!”

Mr McCarthy, an editor at the National Review, wrote an article last week headlined, Did the FBI Have a Spy in the Trump Campaign?

White House spokeswoman Sarah Sanders said if the reports are proved to be true, “it should certainly be looked into”.

Mr Trump also marked the Russia investigation anniversary with a series of Twitter posts, saying “we are now into the second year of the greatest witch hunt in American history”.

Congratulations America, we are now into the second year of the greatest Witch Hunt in American History…and there is still No Collusion and No Obstruction. The only Collusion was that done by Democrats who were unable to win an Election despite the spending of far more money!

— Donald J. Trump (@realDonaldTrump) May 17, 2018

Mr Trump also praised his administration, calling it “the most successful first 17 month Administration in US history – by far!”

Mr Trump’s lawyer Rudy Giuliani told Fox and Friends on Friday (Thursday local time) his client still wants to testify in the Russia probe.

He said Mr Trump would only sit down with Mr Mueller if “we feel there’s a way to shorten this thing”.

-with AP