Swedish authorities said Thursday a fourth leak was detected on undersea pipelines linking Russia to Europe following what NATO described as an act of sabotage.
The Nord Stream 1 and 2 pipelines have been at the center of geopolitical tensions as Russia cut gas supplies to Europe in suspected retaliation against Western sanctions following Moscow's invasion of Ukraine.
The Swedish Coast Guard confirmed Thursday there were four leaks in total on the pipeline in the Baltic Sea — two on the Swedish side and two on the Danish side. Three leaks were previously reported.
NATO declared that the damage was "the result of deliberate, reckless and irresponsible acts of sabotage" and said it supported investigations to determine the origin of the damage.
The Western alliance warned that it was "committed to prepare for, deter and defend against the coercive use of energy and other hybrid tactics."
"Any deliberate attack against Allies' critical infrastructure would be met with a united and determined response," it said, adding that the leaks present risks to shipping and cause substantial environmental damage.
Russia, which has denied it was behind the explosions, said a foreign state was likely responsible for what it described as a "terrorist act."
Russia's security service launched an "international terrorism" investigation into the gas leaks, saying the damage had caused "significant economic damage to the Russian Federation."
Kremlin spokesman Dmitry Peskov said that such a probe "required the cooperation of several countries", but that there was an "unwillingness" from many countries to contact Russia.
Russia said Wednesday that Washington should answer if it was behind the leaks — an assertion rejected by the United States as "ridiculous."
The UN Security Council will meet Friday to discuss the matter.
Finland, which borders Russia, decided to reinforce security around its critical infrastructures, with a particular focus on the electricity network.
Sweden's two main nuclear power plants stepped up their alert levels.
'Constant' gas flow
Operated by a consortium majority-owned by Russian gas giant Gazprom, Nord Stream 1 and 2 run from Russia to Germany.
While the pipelines are not currently in operation, they both still contained gas.
The vast leaks have caused underwater gas plumes, with significant bubbling at the surface of the sea several hundred meters wide, making it impossible to immediately inspect the structures.
Seismic institutes on Tuesday reported they had recorded "in all likelihood" explosions in the area, prior to the leaks being detected.
A Swedish Coast Guard search and rescue vessel was patrolling the area.
"The crew reports that the flow of gas visible on the surface is constant," the agency said in a statement.
Danish authorities said the leaks will continue until the gas in the pipelines is exhausted, which is expected to occur on Sunday.
Fatih Birol, executive director of the International Energy Agency, said at a symposium in Paris that to him it was "very obvious" who was behind the leaks.
He said natural gas shortages in the wake of the war in Ukraine could make for a tough winter in Europe.
"In the absence of a major negative surprise, I think Europe, in terms of natural gas, can survive this winter with a lot of bruises in our bodies in terms of prices, economy and social issues, but we can go through that," Birol said.
According to climate groups, Nord Stream 1 and 2 contained some 350,000 tonnes of natural gas -- methane.
Greenpeace says the leaks could have the effect of almost 30 million tons of CO2, or more than two-thirds of the annual emissions of Denmark.