Denmark and Sweden on Tuesday said major leaks on the two Nord Stream pipelines in the Baltic Sea were caused by deliberate acts of sabotage carried out in each of the two countries' exclusive economic zones.
Neither pipeline was in operation amid an energy standoff between Russia and Europe. Moscow has slashed gas deliveries to Europe after the West imposed sanctions over Moscow's invasion of Ukraine.
The European Union warned of a strong response should any of the bloc's active infrastructure be attacked.
"Any deliberate disruption of European energy infrastructure is utterly unacceptable and will be met with a robust and united response," EU foreign policy chief Josep Borrell said in a statement on Wednesday.
He was echoing a warning by European Commission chief Ursula von der Leyen who attributed the pipeline leaks to acts of sabotage late on Tuesday.
"Any deliberate disruption of active European energy infrastructure is unacceptable and will lead to the strongest possible response," she said on Twitter.
Borrell announced the bloc would step up the protection of its energy infrastructure following the incidents.
"We will support any investigation aimed at getting full clarity on what happened and why, and will take further steps to increase our resilience in energy security," he said.
(Reporting by Sabine Siebold and Bart Meijer)