The operator of the Nord Stream pipelines, built to carry Russian natural gas to Europe, reported "unprecedented" damage to the system, with mysterious leaks in three lines beneath the Baltic Sea.
The leaks had no immediate impact on energy supplies to the European Union, since Russia had already cut off gas flows.
But gas had remained in the pipes, raising concerns about possible environmental harm from leaking methane, the main component of natural gas and, when in the atmosphere, a major contributor to climate change.
Images supplied by the Danish military showed gas bubbles reaching the surface of the water.
European officials said that the damage may have been sabotage.
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