The information includes details of how the NSA penetrates foreign computer networks, the computer code it uses for such spying and how it defends networks inside the US, The Wall Street Journal reported on Thursday, citing unidentified sources.

The information would help the Russian government protect its own networks and make it more difficult for the NSA to conduct its work.

It's an embarrassing breach for the NSA, which has struggled with contractor security since the Edward Snowden leaks in 2013.

Read more: Russia is hacking and harassing NATO

The theft was discovered in the spring of 2016 and is one of the most significant security breaches in recent years. According to the report, the hackers identified the files after an antivirus scan by Russia-based Kaspersky Lab software alerted them to the sensitive files.

The paper reported that the contractor had used antivirus software by Russia-based Kaspersky Lab that made it possible for the hackers to identify and target the contractor's files.

The contractor is not believed to have had malicious intent and was apparently trying to get off-hours work done.

The revelations come amid multiple investigations into Russian hacking of US targets since the 2016 presidential election.

Kaspersky denies

Kaspersky Lab, which has 400 million customers globally, has denied it spies for the Russian government.

The Wall Street Journal quoted Kaspersky Lab saying it "has not been provided any information or evidence substantiating this alleged incident, and as a result, we must assume that this is another example of a false accusation."

The paper said it was the first known incident of Russian hackers using Kaspersky software to spy on Washington.

The Department of Homeland Security banned Kaspersky products in federal networks on September 13 and the Senate passed a bill to ban the program from use by the federal government. There were concerns the company may be used by the Kremlin.

NSA under attack

US Senator Ben Sasse, a member of the Armed Services Committee, chastised the NSA in a statement.

"It's a lot harder to beat your opponent when they're reading your playbook, and it's even worse when someone on your team gives it to them. If these reports are true, Russia has pulled that off," he said. "The NSA needs to get its head out of the sand and solve its contractor problem."

It is not the first such case, after NSA contractor Harold Martin was charged with taking home classified documents in 2016.

The Justice Department also charged a NSA contractor - 'Reality Winner' - with leaking classified documents concerning Russian election interference earlier this year.

jbh/sms (Reuters, AP)