The leak, on one of the two strands of the western section of the pipeline, was found in the city of Wloclawek, 70 kilometers (43.5 miles) from the country’s oil hub Plock in central Poland, PERN said in a press release.

"The cause of the incident is unknown at the moment - pumping in the damaged line was shut down immediately. The other oil pipeline is operating without changes," it said.

"The spill of the semi-finished crude oil from the oil pipeline operated by PERN occurred from a pipe of an average of 80 cm with a pressure of 30 atm,” fire brigade spokesman Karol Kierzkowski told local reporters.

He said firefighters are operating in the vicinity of the village of Zurawice in the neighboring municipality of Boniewo.

"We are still looking at the leak in a nearby cornfield. It is not easy, because everything is obscured by the semi-finished oil that has flowed into the field," Kierzkowski told Poland's PAP news agency on Wednesday morning.

He said the leak will cause damage to the environment.

According to the pipeline operator, other parts of the pipeline, including the Pomeranian section, which pumps crude oil arriving in Poland and then also to Germany, are operating in standard mode.

Druzhba is the world's largest pipeline system, connecting Siberia and Europe.

The pipeline is the main source of oil supplies to Polish refineries, with a total capacity of 56 million tons per year.

Last month, leaks in the Russia-owned Nord Stream 1 and Nord Stream 2 pipelines were reported in the Baltic Sea off the coast of Denmark and Sweden, with ships warned not to approach the area. Both countries, as well as the EU, said there were signs that the leaks were the result of deliberate action and not an accident.

NATO defense ministers in a two-day meeting on Wednesday and Thursday will also address how to further increase the protection of critical infrastructure in light of the sabotage attacks on the Nord Stream gas pipelines.

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