Powerful explosions were recorded by Danish and Swedish seismologists in the areas of the Nord Stream gas leaks, Sweden's National Seismology Center (SNSN) told public broadcaster SVT on Tuesday.
"There is no doubt that these were explosions," SNSN seismologist Bjorn Lund told SVT.
Two spikes were recorded by a seismograph on the Danish island of Bornholm on Monday, the day on which the Nord Stream 1 and 2 gas pipelines underwent dramatic falls in pressure, German geological research centre GFZ said.
The seismograph recorded near-silence until 12:03 a.m. GMT (2 a.m. local time) when there was a spike representing a tremor in the earth followed by a continuous hissing waveform. The pattern repeated itself at 5 p.m.
European politicians and security experts have suggested that the pipeline rupture near Bornholm could have been caused by sabotage. GFZ declined to be drawn on whether the tremors recorded could have been the result of an explosion.
"There was a spike and then regular noise," said GFZ spokesperson Josef Zens. "We cannot say if that could be gas streaming out."
Nord Stream 2 was first reported to have depressurised on Monday afternoon. The depressurisation in the Nord Stream 1 pipeline was reported in the early evening, shortly after the second of the two spikes.
Hard to imagine Nord Stream gas leaks 'coincidence': Danish premier
Danish Prime Minister Mette Frederiksen on Tuesday said that it is hard to imagine that the gas leaks on the Russian Nord Stream pipelines are a "coincidence."
"It is unusual and I would like to say that we from ... the government ... and the authorities are taking this very seriously," the prime minister told a local news outlet DR.
Asked if the three leaks are sabotage, Frederiksen said it cannot be "ruled out."
"It is one ... extraordinary ... situation, and there are three leaks, which is why it is difficult to imagine that it should be random," she said.
The Danish Energy Authority confirmed that the gas leaks from Nord Stream pipelines will continue for days, the daily Berlingske said.
Shortly after discoveries about the leaks on the Russian Nord Stream 2, the Swedish Maritime Authority issued a warning on Tuesday about two leaks on the Russia-owned Nord Stream 1 pipeline in Swedish and Danish waters.
Denmark's energy agency confirmed that three leaks in total are detected on Nord Stream 1 and on Nord Stream 2 gas pipelines, northeast and southeast of the island of Bornholm.
Director of the Danish Energy Agency Kristoffer Botzauw said earlier that breakage of gas pipelines is extremely rare, "and therefore we see reason to raise the preparedness level as a result of the incidents we have seen over the past 24 hours."
The emergency response of the electricity and gas sector is raised to the second highest level, the outlet reported.
He also informed that thorough monitoring of Denmark's critical infrastructure is undertaken to "strengthen the security of supply going forward."
Energy and Supply Minister Dan Jorgensen told the news outlet DR that the leaks on the Nord Stream pipes do not change the security of supply.
"This does not change the immediate situation of security of supply. The gas stocks in Europe are close to full," he said.
The prime minister and the energy minister are currently in Poland in connection with the opening of the Baltic Pipe, another gas pipeline which would send gas from Norway through the Danish territory to Poland.
The Russia-owned Nord Stream 2 pipeline flowing from Russia under the Baltic Sea to Germany was originally created to double the volume of gas, however just as the works completed, Germany decided to stop it, after the start of Russia's war on Ukraine.
Nord Stream 1 pipeline that stretches 1,200 km (745 miles) under the Baltic Sea from the Russian coast near St. Petersburg to northeastern Germany was stopped by Russia following German's decision to cut gas supply from Nord Stream 2.