"The damage to the underwater infrastructure was taken seriously and an investigation into its cause has been ongoing since Sunday … It is likely that damage to both the gas pipeline and the telecommunications cable is the result of external influence," Niinisto said in a statement.
The president added that the cause of the incident remains unknown, and the investigation is ongoing in cooperation between Finland and Estonia.
"We are also in constant contact with our allies and partners. Today I held talks with NATO Secretary General Jens Stoltenberg. NATO is ready to assist in the investigation," Niinisto said.
Timo Kilpelainen, head of the Investigation Department at Finland’s National Bureau of Investigation, said the agency was treating the pipeline damage as a case of "aggravated vandalism."
"One of the possible scenarios at the moment is that of aggravated vandalism," the official said, adding the case was unusual because there was no sign that explosives were used.
Swedish Foreign Minister Tobias Billstrom offered his country’s help in investigating the pipeline leak, saying on social media that Sweden "stands with our Nordic-Baltic colleagues and are ready to offer any help regarding the damage to undersea infrastructure."
The Baltic nation of Lithuania said meanwhile that its newly established National Crisis Management Center would increase ramp up security at strategic facilities.