Nord Stream
© AFP / Danish DefenseGas leak along one of the damaged Nord Stream pipelines
Norway's offshore energy operators reported an increasing number of unidentified drones near their platforms in the days leading up to the apparent sabotage of the Russian Nord Stream 1 and 2 gas pipelines.

The Norwegian Petroleum Safety Authority held a meeting with members of the country's oil and gas industry on Thursday, warning them that recent reports indicated "increased drone activity" over the Norwegian continental shelf (NCS).

"These craft have been of different sizes, and their presence has been growing - particularly in September," the agency stated, adding that the drone sightings are being investigated by the Norwegian police.

In a letter dated last Friday, the agency warned Norway's maritime and aviation authorities that energy operators on the NCS had "recently given warnings/notifications of a number of observations concerning unidentified drones/aircraft close to offshore installations."

Three days after the letter was sent and around 1,000km (620 miles) to the south of the NCS, a leak was reported along the Nord Stream 2 pipeline, followed by two leaks a day later along Nord Stream 1, all near the Danish island of Bornholm.

As of Thursday, a total of four leaks had been discovered along the pair of Russia-to-Germany gas lines, according to the Swedish coast guard.

Sweden and Denmark told the United Nations Security Council on Thursday that "at least two detonations" equal to "several hundred kilos" of TNT took place at the sites of the leaks. Without identifying a culprit, the two Nordic nations wrote that "all available information indicates that those explosions are the result of a deliberate act."

While the Ukrainian government and some US pundits have blamed Russia for attacking its own pipeline, Moscow has suggested Western involvement. "We already have certain materials, which point to a Western trace in organizing and executing this terrorist attack," the director of Russia's Foreign Intelligence Service, Sergey Naryshkin, told reporters on Friday.

US President Joe Biden said in early February that the US would "end" Nord Stream 2 if Russian troops entered Ukraine, telling a reporter "I promise you, we will be able to do it."

Additionally, the US-led NATO alliance conducted military exercises involving underwater drones near Bornholm in June, while US military vessels were spotted near the island last week.