Russian hackers might have access to US electric utilities and could use them to cause blackouts across the country, according to new warnings.

The group – who worked for a Russian state-sponsored group known as Dragonfly or Energetic Bear – gained access to networks last year and the campaign is probably continuing, according to a report in The Wall Street Journal.

The hackers have already claimed “hundreds of victims”, according to officials. But much of their most dangerous work might not actually have happened yet – with cyber attacks gaining access to the most critical parts of US infrastructure and then sitting inside of it.

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Hackers used conventional tools such as spear-phishing emails and watering-hole attacks that trick victims into entering their passwords and then gained access to corporate networks of suppliers, which allowed the hackers to steal credentials and gain access to utility networks, the report said.

The Department of Homeland Security did not respond to a request for comment outside regular business hours.

DHS plans to conduct four briefings and is searching for evidence on the Russians attempting to automate their attacks, the report said. Investigators cited by the WSJ said it was not clear whether this was done by the hackers in preparation for a bigger future attack.

The report comes amid increasing cyber tensions between Moscow and Washington. A federal grand jury in the US indicted 12 Russian intelligence officers earlier in July on charges of hacking the computer networks of 2016 Democratic presidential candidate Hillary Clinton and the Democratic Party.

Special counsel Robert Mueller is investigating Russia’s role in the 2016 election and whether the campaign of Republican candidate Donald Trump colluded with Moscow. Russia denies meddling in the elections while Mr Trump has denied any collusion.