Robert Mueller, the special counsel leading the FBI's Russia investigation. Alex Wong/Getty Images

A spokesman for the office of the special counsel Robert Mueller as said "many" stories about his investigation "have been inaccurate."

It was an unusual piece of pushback from an office that tends to keep quiet.

The progress of Mueller's investigation into Russia and the Trump campaign is a source of fascination for many news outlets.

Many stories are based on leaks from anonymous figures said to have knowledge of the investigation, which are inherently difficult to verify.

The office of the special counsel Robert Mueller has said much of the reporting on the investigation into Russian election meddling and the Trump campaign are inaccurate.

A statement warned reporters to "be very cautious" about claims made of the internal workings on Mueller's investigation, which it said resulted in "many" claims that didn't stack up.

News outlets, including Business Insider, routinely report on the progress of the investigation, often citing anonymous sources whose knowledge of the investigation is often unclear to readers.

Mueller heading in to an off-the-record meeting with lawmakers in Washington, DC, in June. Alex Wong/Getty Images

The practice is a long-standing journalistic convention designed to protect sources from reprisal for speaking out of turn. Mueller's office does not typically give much on-the-record commentary.

But this statement, confirmed by Business Insider, struck a different tone. It was previously reported by The Washington Times, a pro-Trump outlet that routinely takes aim at reports hostile to the president.

The spokesman said:

"What I have been telling all reporters is that many stories about our investigation have been inaccurate.

"Be very cautious about any source that claims to have knowledge about our investigation and dig deep into what they claim before reporting on it.

"If another outlet reports something, don't run with it unless you have your own sourcing to back it up."

Trump has routinely attacked anonymously sourced political reporting, alleging that sources without names attached are often fabricated. For example:

That said, Trump has also promoted anonymously sourced stories that are favorable to him.

And as a New Yorker report from June 2017 illustrated, Trump himself is believed to have a history of serving as an anonymous source, for stories in The National Enquirer.

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