Sweden on Thursday reported a fourth gas leak in the Baltic Sea, widening the scope of the suspected sabotage.
It came as Nato members vowed a "united and determined response" to any deliberate attack against their energy infrastructure.
Germany called on Nato to step in and protect pipelines after the apparent undersea explosions, which Chancellor Olaf Scholz said had closed the door on any imports from Russia for the foreseeable future.
Gas was bubbling to the Baltic surface for a fourth day and was expected to keep pouring out until the two Nord Stream pipelines were empty, which could take more than a week.
Swedish coastguards patrolling the turbulent waters announced on Thursday that a second leak had been found in Sweden’s maritime zone, to add to the two in Danish waters.
The parallel, Russian-controlled Nord Stream 1 and Nord Stream 2 pipelines each have two known leaks after the latest damage was discovered, Swedish media said.
Nato's 30 members said current information "indicates that this is the result of deliberate, reckless and irresponsible acts of sabotage".
"We, as allies, have committed to prepare for, deter and defend against the coercive use of energy and other hybrid tactics by state and non-state actors. Any deliberate attack against allies’ critical infrastructure would be met with a united and determined response," a joint statement said.
Amid speculation of Russian involvement, German diplomat Miguel Berger said a non-state actor could not have caused the apparent blasts. However, he said investigators would have to wait until the gas has stopped leaking.
“We have to wait until all the gas is out because currently it’s too dangerous to investigate, so that means maybe in another 10 days we will know a little bit more,” Mr Berger, Germany’s ambassador to Britain, told BBC Radio 4’s Today programme.
“I think we need to discuss what has happened within Nato and take conclusions for our critical infrastructure. What about our critical infrastructure, undersea cables, pipelines, telephone networks? All of that, I think, needs now additional surveillance.”
Seismographs in Germany, Denmark and Sweden all reported two separate spikes on Monday near where the leaks were discovered. Danish scientists said the tremors did not appear to be natural earthquakes.
Norway, the biggest gas exporter to Europe since Russia supplies dried up, announced it was increasing security on its continental shelf after the Nord Stream leaks and reports of increased drone activity.
A former Polish foreign minister added to the calls for Nato involvement on Wednesday by saying warships, submarines or military aircraft could be sent to guard the Baltic.
While Russia argued it had no interest in blowing up its own pipelines, some interpreted the blast as a warning from Moscow that it could step up its meddling with Europe’s energy supplies.
"We have to be able to deal with, as the last 24 hours have shown, events happening around us that Russia may be behind, that are hybrid in nature, are not to do with overt military conflict but are testing the alliance and indeed the European Union in different ways," said Angus Lapsley, an assistant secretary general of Nato.
Exports through Nord Stream 1 were drastically reduced over the summer and stopped altogether in September, deepening the European energy crisis. Nord Stream 2 was suspended by Germany in February.
Mr Berger said it was not yet known whether the pipelines could ever be repaired and potentially go back into operation.
Russian officials sought to deflect the suspicion by hinting at American involvement, after the US heavily lobbied its European allies not to go ahead with Nord Stream 2.
Kremlin spokesman Dmitry Peskov dismissed the suggestion of Russian involvement and said "many more aircraft and vessels belonging to Nato countries have been spotted in the area".
Russia-friendly accounts recalled a statement by US President Joe Biden in February that Washington would “bring an end” to Nord Stream 2 if Russia invaded Ukraine.
The State Department in Washington described the counter-narrative as Russian disinformation and dismissed the suggestion of US involvement.
“The idea that the United States was in any way involved in the apparent sabotage of these pipelines is preposterous,” said department spokesman Ned Price.
