Russian crude oil supplies to Hungary, the Czech Republic and Slovakia via Ukraine were halted last week after Russian pipeline operator Transneft PJSC was unable to pay transit fees due to sanctions.
Moscow has already blamed the sanctions for curbing natural gas supplies to Europe via the Nord Stream pipeline.
A similar disruption in oil flows could exacerbate the region's energy crisis.
Germany and Poland not affected
Ukrtransnafta, the operator of Ukraine's oil pipeline network that transports crude oil via the southern section of the Druzhba link, stopped deliveries on August 4, Transneft said in a statement Tuesday.
The flows over the northern section of the connection to Poland and Germany are not affected.
The contract between Transneft and Ukrtransnafta provides for 100% upfront payment for transit volumes.
The Russian pipeline operator paid the August transit fee on July 22 and received the money back on July 28, according to Transneft.
According to Transneft, European banks do not have the power to make their own decisions about cross-border payments from Russia and need approval from national regulators.
Transneft is working on alternative ways to transfer the funds, the statement said.
The Ukrainian operator initially did not comment.