EU Commission President Ursula von der Leyen attributes the leaks in the Baltic Sea pipelines Nord Stream 1 and 2 from Russia to Germany to sabotage.
She spoke to Danish Prime Minister Mette Frederiksen about "the act of sabotage," von der Leyen wrote on Twitter on Tuesday evening.
It is now of the utmost importance to investigate the incidents in order to get "complete clarity" on what happened and the background.
"Any intentional disruption of active European energy infrastructure is unacceptable and will result in the strongest possible response," warned the Commission President.
Since Monday, gas has been leaking from the Nord Stream 1 and 2 pipelines from Russia to Germany at three points near the island of Bornholm.
The Danish Navy released images showing large-scale formation of bubbles on the sea surface.
There was initially no official information on a possible cause of the leaks.
An act of sabotage is suspected.
Frederiksen said the Danish government assumes "premeditated actions".
Polish Prime Minister Mateusz Morawiecki spoke of an "act of sabotage" that was "probably the next escalation level" in the Ukraine war.
Sweden's outgoing Prime Minister Magdalena Andersson said there had been "detonations" in the Baltic Sea.
Her foreign minister, Ann Linde, said she didn't want to "speculate about motives or perpetrators."
Although the Nord Stream 1 and 2 pipelines are currently not in operation, they are filled with gas.
Copenhagen assumes that it will take "at least a week" for the methane escaping from the pipes to be used up, said Danish Energy and Climate Minister Dan Jörgensen.
The operating company Nord Stream announced on Tuesday an investigation to work with the local authorities to determine the damage and clarify the causes of the incident.
It is currently not possible to estimate how long it will take to repair the pipelines.