Finland says damage to an underwater natural gas pipeline with Estonia on Sunday may have been deliberate and was probably caused by "external activity".
The Baltic-connector pipeline was shut down after a sudden drop in pressure. A telecoms cable was also damaged.
Finland's prime minister said on Tuesday that the source of the leak had been found and was being investigated by both countries.
Petteri Orpo added that the cause was not yet clear.
Finnish sources have told the BBC the suspicion falls on Russian sabotage as "retribution" for Finland joining Nato in April this year.
Norway's seismological institute, Norsar, said it had detected a "probable explosion" along Finland's Baltic Sea coast at 01:20 on Sunday (22:20GMT on Saturday). The event was measured at 1.0, far smaller than the explosions that targeted the Nord Stream pipelines in September 2022.
That appeared to contradict a statement from Finland's National Bureau of Investigation (KRP) that there were "no indications" that explosives had been used, although it added the damage was so serious it was expected to take months to repair.
Finnish authorities said damage to the cable and pipeline damage happened at two different spots in Finland's Exclusive Economic Zone.
"The discovered damage could not have been caused by normal use of the pipeline or pressure fluctuations," Mr Orpo told a press conference. Other possible causes such as seismic activity had already been ruled out.
The pipeline is Finland's only direct link to the wider European Union's gas network. Nevertheless, Mr Orpo said there were enough alternative sources of gas to ensure the country's energy security was not at risk.
Jens Stoltenberg, the head of the Nato military alliance of which both Finland and Estonia are members, wrote on X (formerly Twitter) that the bloc was "sharing information & stands ready to support Allies concerned".
"Frankly we were expecting something like this sooner," a Finnish source told the BBC's security correspondent, Frank Gardner. However, he says public statements from Helsinki have carefully avoided directing blame at Moscow in case a lengthy inquiry comes up with inconclusive results.
The prospect that the damage could be deliberate has pushed up European gas prices.
UK prices jumped as much as 13.5% on Tuesday to 124 pence per therm (a measurement of gas) - having traded as low as 88 pence on Friday.
Prices were already rising after Israel closed one of its largest gas fields, Tamar, in the Mediterranean Sea, in response to the recent aggression by Hamas.
The damage to the Baltic-connector pipeline has revived concerns about energy security following the Nord Stream pipeline blasts last year.
The Baltic-connector opened in 2020, and is used to send gas between Estonia and Finland, depending on which country is most in need at any point.
The pipeline has been Finland's only natural gas import channel since Russian imports were halted in May last year. Natural gas accounts for about 5% of Finland's energy consumption.
