Russian hackers attacked at least one US voting software supplier days before last year's presidential election, according to a classified government intelligence report leaked on Monday that suggests election-related hacking penetrated further into US voting systems than previously known.

The "Top Secret" National Security Agency (NSA) report, which was published online by The Intercept, does not say whether the hacking had any effect on election results.

It does say, however, that Russian military intelligence attacked a US voting software company and sent spear-phishing emails to more than 100 local election officials at the end of October or beginning of November.

US intelligence agencies declined to comment.

The Intercept's report of the leaked document was published shortly before a government contractor was arrested on suspicion of leaking classified information to an undisclosed news organisation. 

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The document, according to The Intercept, said Russian military intelligence "executed cyber espionage operations against a named US company in August 2016 evidently to obtain information on elections-related software and hardware solutions, according to information that became available in April 2017".

The hackers are believed to have then used data from that operation to create a new email account to launch a spear-phishing campaign targeting US local government organisations, the document said.

"Lastly, the actors [sent] test emails to two non-existent accounts ostensibly associated with absentee balloting, presumably with the purpose of creating those accounts to mimic legitimate services."

The information in the leaked document seems to go further than the US intelligence agencies' January assessment of the hacking that occurred.

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"Russian intelligence obtained and maintained access to elements of multiple US state or local electoral boards," an assessment said earlier this year.

The US Department of Homeland Security "assesses that the types of systems Russian actors targeted or compromised were not involved in vote tallying".

The Intercept contacted NSA and the national intelligence director's office about the document and both agencies asked that it not be published.

US intelligence officials then asked The Intercept to redact certain sections. The Intercept said some material was withheld at US intelligence agencies' request because it wasn't "clearly in the public interest".

The authenticity of the May 5 NSA document, which The Intercept said it obtained anonymously, could not be confirmed.

Reality Winner was arrested for leaking a classified report to an online news organisation [AP Photo via Instagram]

The leaked document was reported on the same day the Justice Department announced it had charged government contractor Reality Leigh Winner, 25, with leaking a classified report containing "Top Secret level" information to an online news organisation. 

The report Winner allegedly leaked is dated May 5, the same date as the document The Intercept posted online.

Winner's lawyer, Titus Thomas Nichols, declined to confirm whether she is accused of leaking the NSA report received by The Intercept.

He also declined to name the federal agency for which Winner worked.

"My client has no [criminal] history, so it's not as if she has a pattern of having done anything like this before," Nichols said in a phone interview Monday. "She is a very good person. All this craziness has happened all of a sudden."

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In affidavits filed with the court, special FBI agent Justin Garrick said the government was notified of the leaked report by the news outlet that received it.

Garrick said the agency that housed the report determined only six employees had made physical copies. Winner was one of them. Garrick said investigators found Winner had exchanged email with the news outlet using her work computer.

Garrick's affidavit said he interviewed Winner at her home Saturday and she "admitted intentionally identifying and printing the classified intelligence reporting at issue" and mailing it to the news outlet.

Asked if Winner had confessed, Nichols said, "If there is a confession, the government has not shown it to me."

Former FBI Director James Comey, who was fired by US President Donald Trump last month, is expected to testify before an intelligence committee this week on the issue of Russian interference in the election.  

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Source: News agencies