As reported by Censor.NET, sources told Reuters that the attacks raised security concerns inside the West's main military alliance, NATO.
Interviews with more than a dozen law-enforcement and private investigators, insiders and utility officials show hackers have quietly made incursions into Baltic networks over the past two years, in parallel with more serious attacks in Ukraine that plunged swathes of that country into darkness.
They say Russian state organizations are suspected of being behind the campaigns.
Reuters could not independently verify the sources' allegations.
At the end of 2015, hackers attacked an Internet gateway used to control a Baltic electricity grid, disrupting operations but not causing blackouts, a source familiar with the matter said. He declined to give details due to ongoing private investigations into the incident, which has not been previously reported.
The source also said suspected Russia-backed hackers had targeted a Baltic petrol-distribution system at around the same time in an unsuccessful denial of service attack that aimed to cause widespread disruption in petrol deliveries.
In a separate malware attack on another undisclosed Baltic grid, also around end-2015, hackers targeted network communication devices, serial-to-ethernet converters (STEC), which link sub-stations to central control, two other sources said. The attack did not cause service disruption, they added.
Though these three incidents date back 18 months or so, cyber security consultants are still investigating some of them. They say hackers can remain dormant and undetected inside systems. In Ukraine, hackers had infiltrated the grids there for about six months before the lights went out in December 2015, consultants said.
STECs were also targeted in Ukraine by the so-called Sandworm team, a Russia-backed group that had attacked energy companies in Western Europe and the United States in a campaign in 2014, several sources said.
The two sources with knowledge of the STEC attacks said they had detected the presence of Sandworm in the Baltics, but they did not give evidence for their suspicion. One of them said Sandworm was still active in the Baltic states.
"It's the same kind of slander as all the other similar accusations," Kremlin spokesman Dmitry Peskov said when asked by Reuters about the possible hacks.
List of Ukrainian companies and agencies whose websites were attacked by hackers on June 27 (live updates)(0)