Fears of sabotage as gas pours into Baltic from Nord Stream 1 and 2 pipelines

Gas is pouring into the Baltic Sea from three separate leaks on the Nord Stream 1 and 2 pipelines amid claims by seismologists in Sweden and Denmark of two sharp spikes in undersea activity, possibly indicating explosions, and speculation about sabotage.

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, the German geological research centre GFZ said.

The seismograph recorded near-silence until just after midnight GMT (2am local time), when there was a spike representing a tremor in the earth followed by a continuous hissing wave form. The pattern was repeated at 5pm GMT.

GFZ declined to be drawn on whether the tremors recorded could have been the result of an explosion but scientists at the research centre ruled out the possibility that the leaks could have been caused by earthquakes.

Earlier, the Danish prime minister, Mette Frederiksen, said sabotage could not be excluded.

The steel pipe itself has a wall of 4.1 cm (1.6 inches) and is coated with steel-reinforced concrete up to 11cm thick. Each section of the pipe weighs 11 tonnes, which goes to 24-25 tonnes after the concrete is applied.

A senior Ukrainian official also called it a Russian attack to destabilise Europe, without giving proof.

British sources said they believed it may not be possible to determine what occurred with certainty.

One UK insider speculated that any explosions were unlikely to have been caused by a submarine or underwater vehicle, because their presence would have been detected in the relatively shallow Baltic waters. Sections of the pipelines are between 80 metres and 110 metres deep.

An alternative scenario could be that mines were dropped from a disguised commercial vessel days or weeks ahead of the attack, then remotely detonated to cause the explosions. But the latter scenario was entirely hypothetical, they stressed.

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. Methane, the primary component of natural gas, partially dissolves in water, is not toxic and creates no hazard when inhaled in limited quantities.

A further drop of pressure was reported on Monday afternoon in Nord Stream 1, which Russia shut down indefinitely at the start of September, initially saying it needed repairs.

Since no gas has flowed through either of the pipelines since the start of the month, German authorities have been quick to reassure people that the leaks will not affect its plan to fill gas storage tanks in time for winter.