In a statement on behalf of all 27 member states, EU foreign policy chief Josep Borrell said: “All available information indicates those leaks are the result of a deliberate act.
“Any deliberate disruption of European energy infrastructure is utterly unacceptable and will be met with a robust and united response.”
Seismologists reported on Tuesday that explosions rattled the Baltic Sea before unusual leaks were discovered on two underwater natural gas pipelines running from Russia to Germany.
Some European leaders and experts pointed to possible sabotage given the energy stand-off with Russia provoked by the war in Ukraine.
The three leaks were reported on the Nord Stream 1 and 2 pipelines, which are filled with natural gas but are not delivering the fuel to Europe.
The damage means that the pipelines are unlikely to be able to carry any gas to Europe this winter even if the political will to bring them online emerged, according to analysts.
Mr Borrell said the EU will support any investigation into the damage, and “will take further steps to increase our resilience in energy security”.
Danish Prime Minister Mette Frederiksen has said that “it is the authorities’ clear assessment that these are deliberate actions – not accidents”.
But she said “there is no information indicating who could be behind it”.
Ms Frederiksen rejected the suggestion that the incident was an attack on Denmark, saying the leaks occurred in international waters.
A seismograph on the Danish island of Bornholm, near where the leaks occurred, twice recorded spikes on Monday, the day on which the Nord Stream 1 and 2 gas pipelines underwent dramatic falls in pressure.
Sweden's national seismic network has said it detected two explosions close to unusual leaks in two Russian natural gas pipelines running under the Baltic Sea. It said the latter explosion was equivalent to a magnitude-2.3 earthquake
Denmark’s defence minister, Morten Bodskov, is meeting Nato Secretary General Jens Stoltenberg later on Wednesday.
US secretary of state Antony Blinken has spoken to Danish foreign minister Jeppe Kofod about the “apparent sabotage”, said US state department spokesman Ned Price who said: “The United States remains united with our allies and partners in our commitment to promoting European energy security,” he said.
An energy standoff over Russia's war in Ukraine halted flows on Nord Stream 1 and prevented supplies from ever starting in the parallel Nord Stream 2.
Kremlin spokesperson, Dmitry Peskov, called the news “very concerning” and said that “no option can be ruled out right now”, including sabotage.
He said: “This is a very concerning news.
"Indeed, we are talking about some damage of an unclear nature to the pipeline in Denmark’s economic zone.”
The steel pipe of the Nord pipeline has a wall of 4.1 cm (1.6 inches) and is coated with steel-reinforced concrete up to 11cm thick.
A five-mile exclusion zone for shipping has been set up around Bornholm, and flights below 1,000 metres have been banned in the area.