It remained far from clear who might be behind any foul play, if proven, on the Nord Stream pipelines that Russia and European partners spent billions of dollars building.
Denmark's Prime Minister Mette Frederiksen said that leaks detected in the Nord Stream gas pipelines clearly were caused by deliberate actions and could not have been a result of accidents.
"It is now the clear assessment by authorities that these are deliberate actions. It was not an accident," Frederiksen told journalists.
"There is no information yet to indicate who may be behind this action," she added.
The major leaks spewed gas into the Baltic Sea near Sweden and Denmark as Sweden launched a preliminary probe into possible sabotage.
Russia, which has slashed gas deliveries to Europe after the US and its allies imposed sanctions over the invasion of Ukraine, said sabotage was a possibility and the incident undermined the continent's energy security.
A senior Ukrainian official claimed it was a Russian attack to destabilise Europe.
The Nord Stream pipelines have been flashpoints in an escalating energy war between European countries and Russia that has pummelled Europe's economies, sent gas prices soaring and sparked a hunt for alternative energy supplies.
Denmark's armed forces released a video showing bubbles boiling up to the surface of the sea.
The largest gas leak had caused a surface disturbance of well over 1km in diameter, the armed forces said.
"Today we faced an act of sabotage, we don't know all the details of what happened, but we see clearly that it's an act of sabotage, related to the next step of escalation of the situation in Ukraine," Polish Prime Minister Mateusz Morawiecki said at the opening of a new pipeline between Norway and Poland.
Seismologists in Denmark and Sweden registered powerful blasts in the areas of the leaks on Monday, Sweden's National Seismology Centre told public broadcaster SVT.
A Swedish seismologist said on Tuesday he was certain the seismic activity detected at the site of the leaks was caused by explosions and not earthquakes nor landslides.
Bjorn Lund, seismologist at the Swedish National Seismic Network at Uppsala University, said seismic data gathered by him and Nordic colleagues showed that the explosions took place in the water and not in the rock under the seabed.
The leaks were very large and it could take perhaps a week for gas to stop draining out of the Nord Stream 2 pipeline, the head of Denmark's Energy Agency Kristoffer Bottzauw said.
"The sea surface is full of methane, which means there is an increased risk of explosions in the area," Bottzauw said.
Kremlin spokesman Dmitry Peskov called it "very concerning news. Indeed, we are talking about some damage of an unclear nature to the pipeline in Denmark's economic zone".
He said it affected the continent's energy security.
Neither pipeline was pumping gas to Europe at the time the leaks were found amid the dispute over the war in Ukraine but the incidents will scupper any remaining expectations that Europe could receive gas via Nord Stream 1 before the northern hemisphere winter.
Meanwhile, Ukrainian presidential adviser Mykhailo Podolyak said Ukraine will not be swayed by any nuclear threats from Russia or annexation votes held on its territory and will press ahead with its plan to retake land from Russia.
Podolyak said the world's nuclear powers should warn Russia that any use of strategic or tactical nuclear weapons in Ukraine would be met with concrete action, and that his country lacked the technology to respond symmetrically itself.
"We will continue our work to de-occupy our territory regardless," he said in an interview.
Podolyak spoke as Russia concluded a fifth and final day of voting in four partially-occupied Ukrainian regions that it says paves the way to their formal annexation.
He derided the votes as legal nonsense.
