Russia has reportedly hacked into U.S. nuclear power plant systems as well as other energy companies, The Washington Post reported Saturday.

The government-backed cyber intrusion appears to be an attempt to size up the capabilities of the energy companies' networks, U.S. government officials told the newspaper.

The officials said the hackers appear to have accessed files related to the sites' business operations, like personnel. There were not signs that the hackers attempted to seize control of or disrupt the power systems.

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The officials expressed concern about the hack, telling the Post that they may conduct additional and more severe cyberattacks on these sites in the future.

The FBI and the Department of Homeland Security warned the energy sector of the hacking threat late last month, Reuters reported at the time, although Russia was not then identified.

The Russian intrusions into these systems is the first known successful case of Russia hacking the nuclear-power-generation sites, multiple U.S. and industry officials told The Post.

The newspaper also reports that the National Security Agency detected the cyber activity by a government-backed Russian spy agency, the FSB. The NSA declined the Post’s requests for comment.

The report comes shortly after President Trump and Russian President Vladimir Putin on Saturday agreed to set up a bilateral cybersecurity working group.

Putin said the group would aim to "prevent interference in the domestic affairs of foreign states, primarily in Russia and the U.S."

The U.S. intelligence community concluded earlier this year that Russia interfered in the 2016 presidential election. Putin has repeatedly denied the allegations.