The Nord Stream pipeline leaks were caused by "at least two detonations" with "several hundred kilos" of explosives, Denmark and Sweden said in a joint letter to the United Nations Security Council.
"At least two detonations occurred underwater, damaging the pipelines belonging to Nord Stream 1 and 2, causing major leaks of natural gas into the Baltic Sea," the countries' permanent missions to the United Nations said.
Leaks were discovered on the two natural gas pipelines earlier in the week, with EU leaders warning that it could be "sabotage".
While the pipelines connecting Russia to Germany still contained some natural gas, the EU said the leaks hadn't affected the bloc's gas supply.
Russia had previously stopped the gas flow to Nord Stream 1, while Nord Stream 2 was not yet operational.
The explosions causing the leaks were recorded by seismological institutes and measured a magnitude of 2.3 and 2.1 on the Richter Scale.
This probably corresponds "to an explosive load of several hundred kilos," Denmark and Sweden's UN missions wrote in the letter.
The leaks occurred in both countries' exclusive economic zones and resulted in natural gas rising to the surface of the water.
The leaks to Nord Stream 1 are expected to continue until 2 October, while an assessment of the Nord Stream 2 leaks is still pending.
"The possible impact on maritime life in the Baltic Sea is of concern, and the climate effect would likely be very substantial due to the significant volumes of greenhouse gas that are being released into the air," they added.
Ships have been warned to maintain a distance of at least five nautical miles (10 kilometres) from the leaks.