A subsea gas pipeline and a telecommunications cable connecting Finland and Estonia have both sustained damage, and the location of the outage has been identified, the Finnish government said on Tuesday.
"It is likely that damage to both the gas pipeline and the communication cable is the result of outside activity," President Sauli Niinisto said in a statement.
"Today I spoke with NATO Secretary-General Jens Stoltenberg. NATO stands ready to assist in the investigations," Niinisto said.
The Baltic Connector gas pipeline was shut early on Sunday on concerns that gas was leaking from a hole in the 77-km (48-mile) pipeline. Finnish operator Gasgrid said it could take months or more to repair.
Prime Minister Petteri Orpo will hold a press conference at 1430 GMT, his office said on Tuesday.
"The fall in pipeline pressure was quite fast, which would indicate it's not a minor breach. But the cause of it remains unclear," said a Baltic energy official with knowledge of the situation, who spoke to Reuters on condition of anonymity.
The pipeline between Inkoo in Finland and Paldiski in Estonia crosses the Gulf of Finland, an arm of the Baltic Sea that stretches eastwards into Russian waters and ends at the port of St Petersburg.
(Reuters - Reporting by Terje Solsvik and Nerijus Adomaitis in Oslo, Anne Kauranen in Helsinki, Anna Ringstrom in Stockholm, Andrius Sytas in Vilnius and Marta Frackowiak in Gdansk; editing by Gwladys Fouche)