As gas continued to spew into the Baltic Sea for a fourth day since leaks were first detected, it remained unclear who might be behind any deliberate attack on the pipelines that Russia and European partners spent billions of dollars building.
Russia said the incidents looked like "an act of terrorism". It has also said the leaks off the coasts of Denmark and Sweden occurred in territory that is "fully under the control" of U.S. intelligence agencies.
"The attack on strategic infrastructure means that the strategic infrastructure in the entire EU has to be protected," an EU official in Brussels said.
"This changes fundamentally the nature of the conflict as we have seen it so far, just like the mobilisation ... and the possible annexation," the official said, referring to Russia's mobilising of more troops for the Ukraine war and expectations President Vladimir Putin will annex Ukrainian regions.
Russia's war with Ukraine and the resulting energy standoff between Moscow and Europe, which has the EU scrambling to find alternative gas supplies, are set to dominate the EU summit on Oct. 7 in Prague.
The European Union on Wednesday warned of a "robust and united response" should there be more attacks and stressed the need to protect its energy infrastructure, but EU officials have avoided pointing a figure directly at possible perpetrators of any sabotage.
The Nord Stream 1 and 2 pipelines were not supplying gas to Europe when the leaks were first detected on Monday but still had gas in them. Russia had halted deliveries via Nord Stream 1, saying Western sanctions had hampered operations. Nord Stream 2 had not started commercial operations.
Next week, EU leaders will discuss an eighth sanctions package on Russia which European Commission chief Ursula von der Leyen has proposed, including tighter trade restrictions, more blacklistings and an oil price cap for third countries.
The EU official said he expected the 27-nation bloc to agree on parts of the sanctions package before the Prague summit, such as the blacklisting of additional individuals and some of the trade restrictions with regard to steel and technology.
Other topics such as the oil price cap or the sanctioning of banks may not be solved before the summit, he added.
EU states need unanimity to impose sanctions and the oil price cap might be too much for Hungary, where Prime Minister Viktor Orban, who cultivates close ties with Russian President Vladimir Putin, has been a vocal critic of economic restrictions.
Fatih Birol, head of the International Energy Agency (IEA), told a news conference on Thursday it was "very obvious" who was behind the suspected sabotage of the pipelines, although he did not name anyone.
"It is not yet known who made it, who is behind this sabotage, there is still discussion more or less but ... it is very obvious ... who was behind this issue," he said in Paris.
Sweden's coastguard discovered a fourth gas leak on the damaged pipelines earlier this week, a spokesperson told the Svenska Dagbladet newspaper.
(Reporting by Sabine Siebold; Editing by Alison Williams and Elaine Hardcastle)
By Sabine Siebold and Charlotte Van Campenhout