Germany is investigating leaks in Nord Stream gas pipelines from Russia.

Authorities were trying to determine the cause of leaks in two gas pipelines running from Russia to Germany under the Baltic Sea after both experienced a sudden drop in pressure, German and Danish officials said.

Neither of the pipelines, Nord Stream 1 and 2, had been active, but both were filled with natural gas when they experienced a sharp drop in pressure on Monday, which authorities said could only be caused by a leak.

“There must be large holes; otherwise pressure would not fall so quickly,” said Fiete Wulff, a spokesman for Germany’s network agency, on Tuesday. “There is no other way to explain it.”

The pipelines have been a focal point of the broader confrontation between Russia and Europe. After the European Union imposed economic sanctions on Russia to penalize it for invading Ukraine, Russia began withholding the natural gas that for decades it had abundantly sent to Europe, threatening its energy supply as winter looms.

Officials in Germany and Denmark said it was not immediately clear what had caused the leaks but that investigations were underway. The leaks would not affect the security of gas supply to either country, they said.

Russia’s Gazprom halted deliveries through Nord Stream 1 indefinitely earlier this month, as part of a continuing dispute with Germany over gas deliveries. Nord Stream 2 was never made operational after Germany canceled its certification on the eve of Russia’s invasion of Ukraine.

Germany’s energy ministry said in a statement Monday that a grid operator had reported “a sharp drop in pressure” in the Nord Stream 2 pipeline. The Danish Maritime Authority said the leak appeared to have occurred off the coast of a Danish island and issued a navigation warning for the area.

Separately, the operator of Nord Stream 1 said it had detected a pressure drop that was later confirmed by the German economy ministry. The pipeline is made up of about 100,000 concrete-coated steel pipes designed to withstand the change in pressure the gas undergoes on the 760 mile journey from Russia to Germany. They lie on the floor of the Baltic Sea.

The head of Germany’s network regulator, Klaus Müller, wrote on Twitter that the country was no longer dependent on Nord Stream 1 but that the development underscored the tense situation around the pipelines.