The Polish operator of the Druzhba oil pipeline that delivers Russian crude to Europe said there was no sign yet that a leak was caused by sabotage.
"Based on first findings and the manner in which the pipeline was deformed, it appears that at this point there are no signs of any third-party interference," PERN said in a statement.
However, a more detailed analysis was being carried out to determine the cause of the damage and to repair the pipeline so that oil can resume flowing on the section at full capacity.
The leak, which was discovered in a field late Tuesday, is around 70 kilometres from the central Polish city of Płock. Technicians, the fire brigade, environmental clean-up teams and investigators were dispatched to the site.
PERN said in a late Wednesday statement that "most of the contamination from the area" has been removed but did not say how much crude had escaped or the size of the hole.
The Druzhba pipeline, whose name means "friendship" in Russian, is one of the biggest crude oil pipeline networks in the world. The section affected delivers Russian petroleum to Germany.
The German government said on Wednesday that the country's oil supplies were secure despite the leak having reduced flows.
The incident came as Europe is struggling with an energy crisis and on alert for signs Russia will further cut flows of fossil fuels amid sky-high tensions over the war in Ukraine.
While the European Union is racing to wean itself off Russian energy sources, and stockpile gas for the winter, it is still buying Russian oil for the moment.
The potential for energy infrastructure to become the target of attacks has spooked the EU. The Nord Stream 1 and 2 natural gas pipelines that run from Russia to Germany were ruptured by still-unexplained blasts late last month, in what European capitals have called cases of suspected sabotage. The pipelines were not in operation.
