Polish operator PERN has detected a leak in one pipeline in the Druzhba system that carries oil from Russia to Europe, it said on Wednesday, an event that will add to concerns about Europe's energy security after the Nord Stream gas leak.

The discovery of the leak PERN said it found on Tuesday evening comes as Europe faces a severe energy crisis in the aftermath of Moscow's invasion of Ukraine, which has cut supplies of gas in a continuing stand-off.

PERN said at this point the causes of the leak are unknown. It was detected in a section of the pipe around 70 kilometers (around 40 miles) from the central Polish city of Plock.

The Druzhba oil pipeline, whose name means "friendship" in Russian, is one of the world's largest, supplying Russian oil to much of Central Europe, including Germany, Poland, Belarus, Hungary, Slovakia, the Czech Republic and Austria.

The second line of the pipeline, and other elements of PERN's infrastructure, were working normally, PERN said.

"At this point, all PERN services (technical, operational, in-house fire brigade and environmental protection) are taking action in accordance with the algorithms provided for this type of situation," the operator said.

A spokesperson for the Czech pipeline operator MERO said it had not seen any change in flows to the Czech Republic.

Germany's Schwedt refinery, which supplies 90% of Berlin's fuel, its Economy Ministry and its federal network regulator were not immediately available for comment.

The leak in the subsea Nord Stream 1 and 2 gas pipelines has been blamed on sabotage by both the West and Russia. It was not immediately clear how significant the leak in the Druzhba pipeline was.