Ukraine has admitted withholding some Russian gas intended for other European countries, but said it will still meet its contractual obligations.
Europeans are concerned about disruption to gas supplies
A spokesman for Ukraine's state-owned energy firm Naftogaz said the excess gas was needed to cope with the especially cold January weather.
Earlier Russia's gas monopoly Gazprom said Ukraine was preventing Russia from meeting its international obligations.
Gazprom supplies a quarter of Europe's gas - most of it via Ukraine.
Supplies have been stretched in recent days amid plummeting temperatures in parts of Europe and Asia.
Ukraine's deep freeze
On Monday the Ukrainian Prime Minister, Yuri Yekhanurov, said his country's consumption of gas had exceeded 400 million cubic metres in the past 24 hours - a record level.
It amounts to about 43% more than during an average winter day.
Temperatures have fallen lower than minus 30C in Ukraine, severely straining its energy infrastructure.
Gazprom said it was compensating for the missing volumes by using underground storage facilities in European countries.
The energy giant said it was extracting nearly 85 million cubic metres of gas above-plan daily to meet the demand.
Tensions persist between Russia and Ukraine over a new gas pricing deal which doubles the amount Kiev has to pay for its gas.
They have postponed the signing of the deal, which was reached on 4 January.
Under the five-year deal, Ukraine will buy Russian and Central Asian gas for $95 per 1,000 cubic metres on average.
The gas dispute - which involved Gazprom temporarily turning off Ukraine's supply - disrupted supplies to a swathe of European countries.