Sergei Grigoriev, a top executive with state-controlled pipeline operator JSC Transneft, blamed oil giant JSC Lukoil and several smaller oil companies and said it was not linked to any repairs of the Druzhba pipeline, which runs from Russia through Belarus en route to Europe.
"The reduction was evident last month and this month," Grigoriev told Dow Jones Newswires.
"Maybe they're looking for another market," he said.
A Lukoil spokesman declined to comment, but said a statement would be released in the coming days.
Meanwhile, a spokesman for the Schwedt refinery confirmed that the plant in eastern Germany has seen disruptions in supplies from Russia since July.
Karl-Heinz Schwellnus, a spokesman for refinery operator PCK Raffinerie GmbH, would not say what was the causing the disruptions, but said PCK has been talking with suppliers to try and ease the disruptions.
Despite the disruptions in Russian supplies, the refinery was operating at full capacity through the use of its own resources, he said.
The refinery is jointly owned by BP PLC, Royal Dutch Shell PLC, Total SA and Agip, and can process approximately 10 billion tons, or slightly more than 9 billion metric tons, a year, about 10 percent of Germany's capacity.
European leaders have had growing doubts about the reliability of Russian oil and gas supplies following several disputes between state-owned Russian energy companies and Ukraine and Belarus, where transit pipelines for European-bound supplies are located.
The disputes resulted in Russian energy exports being disrupted and led to calls in some European capitals for Europe diversify its sources.
Russia and Germany are also building a natural gas pipeline under the Baltic Sea intended to get Russian supplies directly Germany and bypass transit countries.