The office of US Special Counsel Robert Mueller has said self-proclaimed hackers in Russia stole evidence in a bid to discredit its probe of a firm charged with funding a Russian propaganda campaign to interfere in the 2016 presidential elections in the United States.

Prosecutors said in a court filing in Washington on Wednesday that a Twitter handle called @HackingRedstone came online on October 22 to brag it had hacked some of the evidence in the case.

"We've got access to the Special Counsel Mueller's probe database as we hacked Russian server with info from the Russian troll case," the court document quoted the Twitter post as saying.

Prosecutors said the vast majority of the files released by the Twitter account were actually "junk material that has nothing to do with the case" and were included along with real files from the investigation in an attempt to tarnish its investigation.

'Enjoy the reading!'

In February 2018, Mueller indicted 13 Russians and three Russian companies with allegations of tampering in 2016 to support then-Republican candidate Donald Trump, who defeated his Democrat rival Hillary Clinton to become the US's 45th president.

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In all, 34 people have pleaded guilty, been indicted or otherwise swept up in the broader inquiry.

The companies named in the indictment included the Internet Research Agency (IRA), known for its trolling on social media, Concord Management and Consulting LLC, which is said to have provided financial backing for the operation, and Concord Catering.

The Twitter account linked to an online file-sharing portal which it said contained Mueller's documents about the "IRA and Russian collusion".

"Enjoy the reading!" it added.

The data that appeared online was "altered and disseminated as part of a disinformation campaign aimed (apparently) at discrediting ongoing investigations into Russian interference in the US political system," prosecutors wrote.

On the same day, a journalist contacted Mueller's office to report receiving a Twitter message from someone who said they had hacked a Russian legal firm that had received the evidence from Concord's US-based law firm Reed Smith LLP.

The illicit activity outlined by prosecutors illustrates the concerns of US intelligence officials about continuing efforts by Russia to interfere in US politics.

The FBI has found no evidence that US servers were compromised, and the IP address of the account used to publish the materials originated in Russia, prosecutors said.

Concord Management is being prosecuted in Mueller's investigation of US allegations that Moscow meddled in the US democratic process to help Trump win.

Mueller is also investigating whether there was any coordination between members of the Trump campaign and Moscow officials. The Kremlin denies election interference and Trump denies there was any collusion, calling the inquiry a political witch-hunt.

Wednesday's filing in the US District Court for the District of Columbia is the latest in a dispute between prosecutors and Concord's US lawyers over how the defence team may share highly sensitive evidence with Concord's Russian corporate officers.

One of those officers is businessman Evgeny Prigozhin, an associate of Russian President Vladimir Putin. He is known in Russia as "Putin's cook". He has been hit with US sanctions over Russian interference in the 2016 election and is charged alongside his company in the indictment brought by Mueller.

In the indictment, prosecutors said Russian defendants adopted false online personas to push divisive messages, travelled to the US to collect intelligence and orchestrated political rallies while posing as US citizens. Prigozhin was one of the 13 people indicted.

He is not expected to appear in a US court because Russia does not have an extradition treaty with the US. His company Concord did hire US lawyers, however, to fight the charges.

Prosecutors say they do not oppose allowing Concord's corporate officers to see the evidence.

However, they fear US intelligence sources and methods could be compromised if the materials are not reviewed in the US. They have asked the judge not to let Concord distribute the materials electronically to people in Russia.