The latest batch of Hillary Clinton emails included highly classified material from some of the US intelligence community's most secretive programmes, a new report revealed. The Intelligence Community Inspector General I Charles McCullough III sent congressional intelligence committees leaders a letter detailing the findings after reviewing Clinton's emails.

Emails on Clinton's private server were flagged by two government agencies as containing classified information, including some on "special access programmes" that are above the "top secret" classification level. Officials said the emails included relatively "innocuous" conversations by State Department officials about the CIA drone programme, NBC News reported.

In the letter to congressional leaders, McCullough said that investigators had discovered "several dozen emails containing classified information determined by the IC element to be at the CONFIDENTIAL, SECRET and TOP SECRET/SAP information." According to NBC, the letter does not detail whether the former secretary of state sent or received the emails, but past emails containing classified information tended to have been sent to her and not from her.

Senator Dianne Feinstein, the ranking Democrat on the intelligence committee, criticised the original Fox News story about McCullough's letter as being partisan. Feinstein said that "none of the emails that are alleged to contain classified information were written by Secretary Clinton."

The senator, who is backing Clinton's presidential bid, added: "None of the emails that were sent to Secretary Clinton were marked as including classified information, a requirement when such information is transmitted."

Brian Fallon, Clinton's campaign spokesman, said the latest report regarding the emails makes it "absurd to suggest that Secretary Clinton did anything wrong." He added: "Both the sending and the leaking of [the IG's] letter was a reckless and irresponsible act." According to CNN, Fallon also suggested the inspector general has an axe to grind with Clinton.

Both the State Department and Clinton's campaign maintain that no information was mishandled, adding that the information and emails being questioned were retroactively classified. CNN reported that the State Department has also stated that the same information could come from different sources and that not all of it were classified.