Putin voiced the amount of Ukrainian debt for gas

"If our partners do not fulfill agreements, we will have to reduce supplies. What else can we do?" Putin said.

Russian energy giant Gazprom last month quoted Ukraine's outstanding debt at $2.4 billion. Some of the funds have reportedly been repaid, but Gazprom has demanded full payment, and threatened supply cuts. Ukraine transits about 80% of Russia's Europe-bound gas, according to Ria-Novosti.

The countries have also agreed on a gradual shift to market gas prices. Ukraine currently pays $179.5 per 1,000 cubic meters for gas imported from Russia or via Russian territory.