According to the Financial Times, Russia’s natural gas monopoly forwarded its claim to its Ukrainian partner, Naftogaz, ahead of Viktor Yanunovich’s visit to the Davos Economic Forum.

Ukraine buys gas from Gazprom on the take or pay basis, which means it has to pay for the full volume set in the contract. 

Gazprom says Naftogaz imported less natural gas than agreed and demanded Ukraine pays its bill. However, Naftogaz has rejected the claim.

Sources: http://www.rbcdaily.ru http://www.vedomosti.ru