Russia has said it is "possible" that its electrical grid is under cyber-attack by the US.
Kremlin spokesman Dmitry Peskov said reports that US cyber-soldiers had put computer viruses on its electrical grid was a "hypothetical possibility".
His comments came in response to a New York Times (NYT) story which claimed US military hackers were targeting Russian power plants.
The report drew scepticism from experts and a denunciation by President Trump.
In its report the newspaper said American "code" had been deployed inside many elements of Russia's power network.
The Times said this was an escalation of other work the US was doing to combat Russian disinformation and hacking campaigns.
Mr Peskov said President Trump had dismissed the allegations made in the Times, calling them "fake news".
The Kremlin spokesman added: "If one assumes that some government agencies do this without informing the head of state, then of course this may indicate that cyber-war against Russia might be a hypothetical possibility."
He said "vital areas" of Russia's economy were under continuous attack, but it had managed to counter the intrusions so they did no damage.
The NYT story was questioned by Thomas Rid, a political scientist from the Johns Hopkins School of Advanced International Studies, who said it made no sense because "publicity burns capabilities".
The story would prompt Russia to search its power network extensively for malicious code, he said, making it likely that any viruses would be found.
He added that the Russian power grid was big and "exceedingly complex" making it very hard for cyber-attackers to get in and leave any virus in place for a long time.
The malicious code was reportedly inserted by soldiers of the US Cyber Command. This group of military hackers is permitted to carry out "clandestine military activity" on computer networks under the aegis of the National Defense Authorization Act, which was passed in 2018.
The US has been probing Russian power systems since 2012, reported the NYT, but was now more interested in finding weaknesses and inserting viruses.