A Swedish prosecutor leading a probe into the sabotage of the Nord Stream pipelines carrying gas from Russia to Germany said Thursday it was "still unclear" who was responsible.

Four large gas leaks were discovered on Nord Stream's two pipelines off the Danish island of Bornholm at the end of September, with seismic institutes recording two underwater explosions just prior to that.

The pipelines had been at the centre of geopolitical tensions as Russia cut gas supplies to Europe in suspected retaliation to Western sanctions over Moscow's invasion of Ukraine.

Washington and Moscow have both denied involvement and each has blamed the other, but public prosecutor Mats Ljungqvist said in a statement it was "still unclear" who was behind the sabotage.

"It's a difficult case, it's a complex case" Ljungqvist told AFP, stressing that the "crime scene is in the Baltic Sea at a depth of 80 metres (263 feet)".

The prosecutor added that those responsible must have done it "knowing full well that they would leave traces behind".

"Our primary assumption is that a state is behind it," the prosecutor explained. Traces of explosive have already been found.

Russian energy giant Gazprom holds a majority stake in the twin pipelines, with the rest owned by German, Dutch and French companies.

Although the pipelines were not in operation when the leaks occurred, they both still contained gas which spewed up through the water and into the atmosphere.