Any deliberate disruption to the EU's energy infrastructure would meet a "robust and united response", its top diplomat said, after several states said two Russian pipelines to Europe that have been spewing gas into the Baltic had been attacked.
It remained far from clear who might be behind the leaks or any foul play, if proven, on the Nord Stream pipelines that Russia and European partners spent billions of dollars building.
Russia, which slashed gas deliveries to Europe after the West imposed sanctions over Moscow's invasion of Ukraine, has also said sabotage was a possibility and that the leaks undermined the continent's energy security, reports Reuters.
The European Union believes sabotage probably caused the leaks detected on Monday in the Nord Stream pipelines, Josep Borrell was reported as saying by German broadcaster ntv, echoing views aired by Germany, Denmark and Sweden on Tuesday. The EU has not named a potential perpetrator or suggested a reason behind it.
"Any deliberate disruption of European energy infrastructure is utterly unacceptable and will be met with a robust and united response," Borrell said, according to ntv.
Denmark's defence minister, meanwhile, said on Wednesday there was reason to be concerned about the security situation in the Baltic Sea region following a meeting with NATO General Secretary Jens Stoltenberg in Brussels.
"Russia has a significant military presence in the Baltic Sea region and we expect them to continue their sabre-rattling," Morten Bodskov said in a statement.
In a sign of how long it might take to ascertain the full extent of the damage and the cause of the leaks, Bodskov also said it might take a week or perhaps two before the areas around the damaged pipelines were calm enough to be investigated.
Sweden's Prosecution Authority said it will review material from a police investigation into the damage to the pipelines and decide on further action.
Swedish Prime Minister Magdalena Andersson had told a news conference on Tuesday that two blasts had been detected in relation to the leaks. Although this did not represent an attack on Sweden, Stockholm was in close contact with partners such as NATO and neighbours such as Denmark and Germany.
Seismologists in Denmark and Sweden said they had registered two powerful blasts on Monday in the vicinity of the leaks and the explosions were in the water, not under the seabed.
The Nord Stream pipelines have been flashpoints in an escalating energy war between capitals in Europe and Moscow that has damaged major Western economies, sent gas prices soaring and sparked a hunt for alternative supplies.
Denmark's armed forces said the largest gas leak had caused a surface disturbance of well over 1 kilometre (0.6 mile) in diameter, while agencies issued warnings to shipping to avoid the area.
Norway has said it will strengthen security at its oil and gas installations in the wake of the leaks and reports of drone activities in the North Sea and Danish authorities asked that the level of preparedness in its power and gas sector be raised.
Operator Nord Stream has called the damage "unprecedented", while Gazprom, the Russian-controlled company with a monopoly on its gas exports by pipeline, declined to comment.
Neither pipeline was pumping gas at the time the leaks were found, but the incidents scupper any remaining expectations that Europe could receive fuel via Nord Stream 1 before winter, while there are also concerns about gas piped via Ukraine.
"A development that could have a more immediate impact on gas supplies to Europe was a warning from Gazprom that Russia could impose sanctions on Ukraine's Naftogaz due to ongoing arbitration," analysts at ING Research said.
Naftogaz's CEO said on Wednesday the Ukrainian energy firm will continue with arbitration proceedings against Gazprom over Russian natural gas which transits the country.
Gazprom said earlier in the week that while rejecting all Naftogaz's claims in arbitration, it may introduce sanctions against the company in case it presses ahead with the case.
"The risk is that these flows come to a complete halt, which will only tighten up the European market further as we move towards the heating season," the ING analysts added.
European gas prices rose following news of the leaks. The benchmark October Dutch price was up by 11% at 204.50 euros/megawatt hour on Wednesday. Although prices are still below this year's peaks, they remain more than 200% higher than in early September 2021.
Russia reduced gas supplies to Europe via Nord Stream 1 before suspending flows altogether in August, blaming Western sanctions for causing technical difficulties. European politicians say that was a pretext to stop supplying gas.
The new Nord Stream 2 pipeline had yet to enter commercial operations. The plan to use it to supply gas was scrapped by Germany days before Russia sent troops into Ukraine, in what Moscow calls a "special military operation," in February.