Ukraine's state antimonopoly agency has imposed a $3.5 billion fine on Russia's Gazprom for allegedly abusing its monopoly control of Ukraine's natural-gas transit system.
Yuriy Terentyev, head of Kyiv's Antimonopoly Committee, made the announcement in a post on Facebook on January 22, but he did not detail Gazprom's violations.
Earlier this week, Gazprom informed Ukraine it owes $2.55 billion for gas supplied in the third quarter of 2015 and that Kyiv has 10 days to pay.
Ukraine disputes the charges, saying a contract clause requiring Kyiv to pay for gas it does not use is "unlawful and void."
Gazprom and the Ukrainian Naftohaz firm have taken the dispute to an international arbitration court in Sweden. Gazprom claims Ukraine owes a total of $29.2 billion.
The European Union gets about 40 percent of the gas it purchases from Russia via the transit network in Ukraine.
Based on reporting by Reuters and Interfax