Finnish President Sauli Niinisto said on Tuesday a leak leading to the shutdown of a natural gas pipeline from Estonia to Finland this weekend was probably caused by "external" activity.

Last year there were a series of underwater blasts that ruptured three pipelines that carried natural gas from Russia to Western Europe, at a time of high geopolitical tensions as Moscow cut gas supplies to Europe.

While the government cautioned against leaping to any conclusions, saying it seemed unlikely that explosives were the cause, the Norwegian seismological institute NORSAR said Monday it detected a "probable explosion" in the area of the leak.

Niinisto said that he had been in touch with NATO secretary-general Jens Stoltenberg, and that the military alliance was ready to assist in the investigation.

He said the cause of the damage was not yet clear, and that Finland and Estonia were cooperating in an ongoing investigation.

Estonian officials confirmed Monday that underwater telecommunications lines linking to Finland were also damaged.

The two damaged elements "are in very different locations, although the timing (of the incidents) is quite close", Estonian Defence Minister Hanno Pevkur told a press briefing.

Finnish Prime Minister Petteri Orpo told reporters later that a preliminary assessment suggested that "the discovered damage could not have been caused by normal use of the pipeline or pressure fluctuations".

He said that the leak was located in the Finnish Exclusive Economic Zone.

When asked about the likelihood of Russian involvement, Orpo said it was important to "gather all the information that is available and not to jump to conclusions at this stage."

Head of the investigation department at the National Bureau of Investigation, Timo Kilpelainen, said at the same press conference that there were "no indications that explosives were used in the act."

NATO chief Stoltenberg posted on X, formerly known as Twitter, that the military alliance was "sharing information and stands ready to support Allies concerned."

European Council President Charles Michel also offered his "full support and solidarity" to the affected countries and called for "a full investigation" in a message on X, formerly Twitter.

'Unusual drop in pressure' 

Finland's gas network operator, state-owned Gasgrid, said Sunday that the Balticconnector pipeline had been shut down over a suspected leak.

The operator said "an unusual drop in pressure" had been noticed in the pipeline running along the seabed of the Gulf of Finland.

Gasgrid added that the Finnish gas system was stable, with supply secured through a floating liquefied natural gas terminal (LNG) in Inkoo.

In late September 2022, a series of underwater blasts ruptured three of the four pipelines that make up Nord Stream 1 and Nord Stream 2, a major conduit for Russian natural gas exports to Western Europe, spewing gas into the Baltic Sea.

The pipelines had been at the centre of geopolitical tensions as Russia cut gas supplies to Europe in suspected retaliation to Western sanctions over Moscow's invasion of Ukraine.

Because the leaks occurred in their exclusive economic zones, Denmark and Sweden opened probes into the attack, as did Germany.

All three countries have kept a tight lid on their investigations, which analysts say is unsurprising given the potential diplomatic fallout of what they might uncover.

Inspection underway 

The Balticconnector has been the only gas import channel to Finland, apart from LNG, since Russian imports were halted in May 2022, following Moscow's invasion of Ukraine.

Gasgrid said Tuesday it had begun its inspection of the pipeline by conducting measurements "with the purpose of determining the location and extent of the predicted damage," adding that the inspection would be carried out in stages during the week.

It added that once measurements were completed repair plans for the pipeline could be made.

Despite the shortfall, Gasgrid said the Inkoo LNG terminal had "the capacity and the ability to deliver the gas Finland needs in the coming winter as well", even though consumption is usually higher in winter.

Telecom operator Elisa also confirmed Tuesday it was experiencing a disruption caused by a "cable break" during the weekend.

"The disruption has no impact on Elisa's services as it is a backup connection," the operator said in a statement.

Russia stopped supplying gas to Finland after it refused to pay in rubles, a condition imposed on "unfriendly countries" — including European Union member states — as a way to sidestep Western financial sanctions against Russia's central bank.

Natural gas accounts for around five percent of Finland's energy consumption, mainly used in industry and combined heat and power production.