New footage of the Nord Stream gas leak shows a bubbling plume of gas erupting from the Baltic Sea after a suspected act of underwater sabotage.
The escaped gas was foaming to the Baltic surface for a third day on Wednesday after three leaks were detected on Russian-controlled pipelines in northern Europe.
A Swedish coastguard vessel built to withstand contaminated environments was monitoring a column of gas more than 950 metres in diameter at the source of one of the leaks.
The coastguard said it could make divers and underwater craft available to help establish what happened, after three apparent blasts deemed unlikely to have been a coincidence.
The Danish frigate Absalon and pollution control ship Gunnar Thorson were enforcing an exclusion zone for shipping around the part of the damage that took place in Denmark's waters.
Russia denied speculation on Wednesday that it was behind the leaks on the parallel Nord Stream 1 and Nord Stream 2 pipelines, both majority-owned by Russian exporter Gazprom.
Denmark said it could be two weeks before the waters calm sufficiently to investigate further. The European Union said the damage to the pipes appeared to be deliberate.
"Pipes don't just leak catastrophically, suddenly," said Prof Joan Cordiner, an engineer at the University of Sheffield. "Typically normal leaks due to corrosion start small and build up over time. Therefore, such a sudden large leak can only have come from a sudden blow cutting the pipe."
Scientists raised concern about the large-scale leak of methane, a highly potent greenhouse gas and the main component of natural gas.
The Swedish coastguard said it was not currently possible to prevent the escape of gas but that some of it was dissolving into the water and air, mitigating the environmental risks.