"If it is proven to be a deliberate attack on NATO critical infrastructure, then this will be serious, but it will also be met by a united and determined response by NATO," Stoltenberg told reporters ahead of a meeting of the NATO defense ministers in Brussels.
The official also stated that he was working in close contact with Finnish President Sauli Niinisto and Estonian Prime Minister Kaja Kallas and stressed the need to establish the reasons for the incident.
Balticconnector is a 48-mile-long undersea gas pipeline connecting the Finnish city of Inga and Estonia's Paldiski. It was shut on October 8 after a sudden drop in pressure raised concerns over a possible gas leak. On Tuesday, Finnish border guards discovered a gas leak. Helsinki believes the damage was most likely caused deliberately.