NATO on Thursday said leaks from the Nord Stream gas pipelines appeared to be sabotage and vowed a "determined response" to any attacks on their critical infrastructure.
"All currently available information indicates that this is the result of deliberate, reckless and irresponsible acts of sabotage," the Western military alliance said in a statement.
"These leaks are causing risks to shipping and substantial environmental damage. We support the investigations underway to determine the origin of the damage," it said.
The statement said that the alliance 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.
Sweden's coastguard on Thursday said a fourth leak had been detected in the pipelines running from Russia to Europe after explosions were reported earlier this week.
The vast leaks come as the West and Moscow square off over the conflict in Ukraine and the Kremlin is accused of using its mammoth energy stocks as a weapon to pressure Europe.
The leaks have caused underwater gas plumes, with significant bubbling at the surface of the sea several hundred metres wide, making it impossible to immediately inspect the structures.
Russia has denied it was behind the explosions -- as did the United States, saying Moscow's suggestion it would damage the pipeline was "ridiculous".
The UN Security Council will meet Friday to discuss the matter.
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