Former US Special Counsel Robert Mueller has been grilled at two congressional hearings in Washington DC about Alexander Downer’s role in sparking the Russian influence probe into Donald Trump’s presidential election campaign.

The questioning centred on a meeting at a London bar in May 2016 between Mr Downer, then Australia’s high commissioner to the UK, and Mr Trump’s campaign aide, George Papadopoulos.

Mr Mueller was also quizzed about a mysterious Maltese diplomat, Joseph Mifsud, who Mr Papadopoulos claims is the person who told him Russia had dirt on Mr Trump’s presidential rival Hillary Clinton.

“So Downer conveys a rumour he supposedly heard about a conversation between Papadopoulos and Joseph Mifsud,” Devin Nunes, the top Republican on the House Intelligence Committee, told Mr Mueller on Wednesday.

“(Former FBI director) James Comey has publicly called Mifsud a Russian agent.

“Yet your report does not refer to Mifsud as a Russian agent.

“Misfud has extensive contacts with Western governments and the FBI.

“For example, there was a recent photo of him standing next to Boris Johnson, the new prime minister of Great Britain.

“What we are trying to figure out here, Mr Mueller, is if our NATO allies or Boris Johnson have been compromised.”

The hearing was shown a photo of Mr Mifsud standing with Mr Johnson.

Mr Mueller’s much-anticipated final report, released in April, stated the London bar meeting between Mr Downer and Mr Papadopoulos prompted the FBI on July 31, 2016 to open its probe into the Trump campaign’s links to Russia.

Mr Downer told The Australian newspaper last year Mr Papadopoulos said during the drinks session Russia had damaging material on Mrs Clinton.

Mr Papadopoulos denies telling Mr Downer this.

Mr Papadopoulos, a 31-year-old from Chicago, pleaded guilty last year and was sentenced to 14 days’ prison for lying to the FBI about his contact with Russian nationals and Mr Mifsud.

Mr Papadopoulos has claimed Australian, UK and US intelligence agencies targeted him in the lead-up to Mr Trump’s election win.

Mr Downer has strongly rejected claims he was spying on Mr Papadopoulos.

Republicans peppered Mr Mueller with questions about Mr Downer and Mr Papadopoulos on Wednesday, but Mr Mueller largely avoided providing answers.

Ohio Republican Jim Jordan listed Mr Downer, Australian official Erika Thompson, Mr Mifsud and Cambridge Professor Stefan Halper among “human sources” the FBI used to “swirl around” Mr Papadopoulos.

Mr Papadopoulos has also claimed a woman, Azra Turk, was sent to spy on him.

“Names like Halper, Downer, Mifsud, Thompson meeting in Rome, London, all kinds of places,” Mr Jordan told Mr Mueller.

“The FBI even sent a lady posing as somebody else, went by the name Azra Turk.

“Even dispatched her to London to spy on Mr Papadopoulos.

“In one of these meetings, Mr Papadopoulos is talking to a foreign diplomat (Mr Downer) and he tells the diplomat, Russians have dirt on Clinton.

“That diplomat then contacts the FBI and the FBI opens an investigation based on that fact.”

-AAP