The EU said Wednesday that leaks from two Russia-Germany undersea gas pipelines appeared to be "a deliberate act", as fossil fuel-rich Norway boosted security at its installations.
The three outflows from the Nord Stream 1 and 2 pipelines have sent natural gas prices soaring, exacerbating an energy crunch in Europe as it stands on the threshold to winter.
Methane gas from the leaks are bubbling to the surface of the Baltic Sea close to Denmark and Sweden in discharges expected to last for a week, until depletion of the gas in the pipelines.
Europe suspects the leaks to be from sabotage.
They "are not a coincidence," EU foreign policy chief Josep Borrell said in a statement. "All available information indicates those leaks are the result of a deliberate act."
He warned: "Any deliberate disruption of European energy infrastructure is utterly unacceptable and will be met with a robust and united response."
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Suspicion has focused on Russia, which has cut gas supplies to Europe in retaliation for severe Western sanctions over the war in Ukraine.
But the Kremlin hit back, saying it was "stupid and absurd" to accuse Russia of causing the leaks.
EU chiefs Ursula von der Leyen and Charles Michel have both blamed the Nord Stream leaks on sabotage.
Michel on Wednesday tweeted that they "appear to be an attempt to further destabilise energy supply to EU".
He added: "Those responsible will be held fully accountable and made to pay."
The EU is currently mulling further sanctions on Russia for annexation votes imposed on four regions in Ukraine its forces occupy.
Weeks before inspection
Neither of the Nord Stream pipelines are currently operational, but they were full with gas when they were hit with what Swedish seismologists said were "massive releases of energy".
One of the seismologists told AFP "there isn't much else than a blast that could cause it".
Danish Defence Minister Morten Bodskov told reporters in Brussels that "it can easily take one or two weeks for the area to calm down enough" for an inspection to verify the cause.
Two Danish military vessels have been sent to the area.
Non-EU member Norway, which has now overtaken Russia as the biggest supplier of gas to Europe, said it will beef up security around its oil and gas facilities.
"The government has decided to put measures in place to increase security at infrastructure sites, land terminals and platforms on the Norwegian continental shelf," Norwegian Energy Minister Terje Aasland said.
The Norwegian Petroleum Safety Authority earlier this week called for "increased vigilance from all operators and shipping companies on the continental shelf".
Built in parallel to the Nord Stream 1 pipeline, Nord Stream 2 was intended to double the capacity for Russian gas imports to Germany.
But Berlin blocked newly completed Nord Stream 2 in the days before the war.
Germany, which has been highly dependent on imports of fossil fuels from Russia to meet its energy needs, has since come under acute stress as Moscow's supplies dwindle.
Sweden and Poland agree sabotage is the most likely cause of the Nord Stream leaks, with Warsaw suggesting Russia was probably the culprit, to escalate the war in Ukraine.
The United States said it was looking into the leaks, with Secretary of State Antony Blinken telling reporters that, if sabotage were confirmed, "that's clearly in no one's interest".
The US has pledged to help European allies on energy security, and is already shipping what liquified natural gas it has to spare to the EU.