Earlier in the day Ukraine's government ordered the Foreign Ministry and national gas company Naftogaz to start talks on direct gas supplies from Russia, bypassing RosUkrEnergo. The country's new prime minister, Yulia Tymoshenko, has pledged to remove "corrupt" intermediary firms from the country's gas deals and to raise transit fees.
The Swiss-registered trader said in a statement: "RosUkrEnergo AG is continuing to carry out the current contract for gas supplies to Ukraine diligently, on time and in full. This was the case in November and December 2007, as well as in January."
"No other company is currently supplying natural gas to Ukraine," RosUkrEnergo said.
The firm transports Turkmen and Russian gas to Ukraine. It has been Ukraine's sole supplier since the start of 2006, when Moscow and Kiev resolved their pricing dispute, which led to a brief cut-off of supplies affecting Europe-bound exports.
The bulk of Russia's gas supplies to the European Union, which account for one quarter of the bloc's demand, runs through Ukraine.
In December, the two states agreed that a mixture of Russian and cheaper Central Asian gas for Ukraine to be sold via RosUkrEnergo would cost $179.5 per 1,000 cu m in 2008 and Russia would pay Ukraine $1.7 per 1,000 cu m for the transit of gas per 100 km, up from the previous $1.6.
RosUkrEnergo later said it would sell Russian gas to Ukraine in January-March for $314 per 1,000 cu m to cover a shortfall in Central Asian supplies.
Since the start of this year, the former Soviet allies have been involved in a fresh dispute over an alleged Ukrainian gas debt.
Ukraine signed an agreement with the World Trade Organization in Geneva on Tuesday opening the way for the country's membership in the global trade body in summer. Experts say this could complicate Russia's gas talks with the country and its own WTO bid.