The temperature is rising in the latest gas row between Ukraine and Russia.
Ukraine wants a discount on the price it pays its neighbour for gas. But Russian President Dmitri Medvedev said Kiev was offering nothing in return, accusing it of trying to sponge off Moscow.
Ukraine is seeking to renegotiate a 2009 deal with Russia. Former Ukrainian Prime Minister Yulia Tymoshenko is on trial over the agreement, which her critics say saddled the country with too high a price for gas.
Moscow says prices will be cut only if Ukraine joins a regional customs union or sells its pipeline grid to Russia. Previous gas rows between the two countries have disrupted supplies to Europe.