Washington (AFP) - US President Donald Trump sharpened Sunday his accusations that his presidential campaign was "infiltrated" or spied on for political purposes, saying he would officially "demand" a Justice Department investigation.

The president has repeatedly cast the probe led by Special Counsel Robert Mueller into Russian meddling in the 2016 election and possible collusion by his campaign as a politically driven "witch hunt," while offering no evidence.

"I hereby demand, and will do so officially tomorrow, that the Department of Justice look into whether or not the FBI/DOJ infiltrated or surveilled the Trump Campaign for Political Purposes," he tweeted, "and if any such demands or requests were made by people within the Obama Administration!"

Trump's angry tweet came amid building pressure from the year-old investigation and reports that the Federal Bureau of Investigation in July 2016 sent a Britain-based American professor to speak separately with three Trump campaign advisers.

A New York Times report described the professor as "an informant" but not a spy, saying the federal agency was looking into evidence that Carter Page, Sam Clovis and George Papadopoulos had suspicious contacts with Russia.

The Washington Post called him "a longtime US intelligence source," used by both the FBI and the CIA.

But Trump and his supporters have cast the man as a mole possibly sent by the Obama administration to burrow into his campaign. "If so, this is bigger than Watergate!" Trump tweeted on Thursday.

While Trump allies in Congress have demanded more information about the informant, law enforcement officials have refused, saying the source -- not yet officially identified -- could be in danger if named.

Democrats say the president's real objective is to undermine the Mueller inquiry.

Trump lawyer Rudolph Giuliani, a former Justice Department prosecutor and assistant attorney general, told CNN on Friday that it was not clear whether an FBI informant had been embedded in the campaign, while adding that the spy agency "should tell us if there was."