The Gazprom-led operator of the Nord Stream 1 gas pipeline has brought its own vessel to Sweden’s Baltic Sea waters as part of its own investigation into the explosions last month that damaged the pipeline, which carries Russian gas to Germany.

The Nord Stream 1& 2 pipelines were both damaged in explosions last month, in what authorities in Sweden and Denmark believe was a deliberate act of sabotage, leading to the largest recorded instance of the escape of highly pressurised natural gas into the atmosphere.

Though the Nord Stream 1 operator — named Nord Stream — has not identified the vessel, tracking websites revealed just one Russian-owned ship, the offshore supply vessel Nefrit, sailed this week from a location near St Petersburg towards the accident northeast of the Danish Island of Bornholm.

Built in Norway in 2004 and previously operated under the name Bourbon Jade, Nefrit was purchased last year by privately held tug service provider Alyans, incorporated in St Petersburg.

“After carrying out calibration works, the specialists will be ready within 24 hours to start the survey of the damaged area that would take three to five days according to current estimates,” Nord Stream said.

The operator added that while it had received permission from Sweden to survey the damage in the Swedish zone, it is still awaiting approval from Denmark.

The Swedish Armed Forces submarine rescue vessel HMS Belos remains in the Swedish waters close to the accident site, which is still being avoided by commercial vessels, according to vessel tracking providers.

HMS Belos arrived on location earlier in October after bubbles of escaping gas stopped appearing on the sea surface.

Two warships, one from Sweden and another from Denmark, also remain on standby close to the accident site.

Before the country’s recent elections, Sweden’s prime minister Magdalena Andersson said her country will not share the findings of its investigation into the Nord Stream 1 & 2 explosions with Russian authorities or Gazprom, provoking strong criticism from Russia and the gas giant.

Sweden’s Security Police said in a statement on Friday that prosecutors decided to conduct additional investigations of the gas pipelines after earlier crime scene investigations had strengthened “suspicions of gross sabotage”.

Russian President Vladimir Putin has previously described Western claims that Russia was behind the explosions as “crazy”, while Gazprom executive chairman Alexei Miller has called them “act of international terrorism”, and blamed the West for the blasts, according to Reuters.

The Nord Stream 1 pipeline consists of two strings — one of which was damaged by an explosion in the Swedish zone of the Baltic Sea, while the other was damaged by an explosion in the Danish zone.

The operator has not said whether Nefrit intends to proceed southeast of Bornholm to survey the damage to the first string of the Nord Stream 2 pipeline, which was ruptured on the same day as the Nord Stream 1 explosions.

While Gazprom and Putin have offered to start sending gas to Germany via Nord Stream 2’s second string, German authorities do not believe the line is safe to operate, according to German magazine Spiegel.

“It is very likely that the act of sabotage… had a negative impact on both pipeline strings and that the basic technical availability is therefore no longer available,” the magazine cited the federal government as saying.