Finnish President Sauli Niinisto said on Tuesday a leak leading to the shutdown of a natural gas pipeline this weekend was probably caused by "external" activity, sparking suspicions Russia could have been involved.

"The important thing now is to establish what happened and how this could happen," Stoltenberg said, at a meeting of NATO defence ministers in Brussels.

"If it is proven to be a deliberate attack on NATO critical infrastructure, then this will be of course serious, but it will also be met by a united and determined response from NATO."

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

While Helsinki 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.

The potential sabotage of the pipeline comes a year after explosions that damaged the Nord Stream pipelines cut off a major route for Russian gas exports to Europe and fuelled geopolitical tensions already running high over Moscow's invasion of Ukraine.

The Kremlin's war pushed Finland to drop its long-standing policy of non-alignment and join NATO, in a major strategic shift that angered neighbour Russia.