It is still unclear who was responsible for the attack on the Nord Stream pipelines last year, as national investigations into the sabotage need to be concluded, NATO's Secretary-General Jens Stoltenberg said on March 8.

"What we do know is that there was an attack against the Nord Stream pipelines, but we have not been able to determine who was behind it," Stoltenberg said before a meeting with European Union defence ministers in Stockholm.

"There are ongoing national investigations and I think it's right to wait until those are finalized before we say anything more about who was behind it."

The New York Times reported on March 7 that new intelligence reviewed by U.S. officials had indicated that a pro-Ukrainian group sabotaged the Nord Stream pipelines that carried natural gas from Russia to Europe.