The damage to the gas pipeline running between Finland and Estonia across the Baltic Sea that burst on Sunday was caused by "quite a heavy force," Estonian Defense Minister Hanno Pevkur said on Wednesday, a day after Helsinki said it could have been a deliberate action.

The undersea Balticconnector gas pipeline and a telecommunications cable connecting Finland and Estonia were damaged on Sunday.

On Tuesday, Helsinki said the damage was likely caused by "outside activity" and that the cause was being investigated.

"It can clearly be seen that these damages are caused by quite heavy force," Pevkur told Reuters, adding that investigators were not ruling out anything at this stage.

"So what it is exactly, we have to specify yet, but at the moment, it rather seems that it had been mechanical impact or mechanical destruction."

Henri Vanhanen, research fellow at the Finnish Institute for International Affairs, said the central issue would be how NATO reacted if evidence was gathered that a state actor was behind the pipeline damage. Finland and Estonia are both members.

"Because this could mean that this is an attack against NATO member infrastructure," he said. "I think the big question in the long term is that, do we have a clear set of potential countermeasures for such activities? What is the deterrence?"

Finnish President Sauli Niinisto said Tuesday that he had been in touch with NATO Secretary-General Jens Stoltenberg and that the military alliance was ready to assist in the investigation.

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

While Finland 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 leak area.

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 Defense Minister Hanno Pevkur told a press briefing.

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

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.