"The situation is critical: We may not have a contract by January 1," Sergei Kupriyanov said at a Moscow-Kiev video press conference at RIA Novosti.
He said Kiev should guarantee Russian natural gas supplies via Ukraine reach Europe, even if an agreement on the transit rate for 2009 has not been agreed upon.
"The transit rate for next year is being discussed, but the Ukrainian side's responsibilities include uninterrupted transit," Kupriyanov said.
He said Ukraine's Naftogaz will not pay Russian energy giant Gazprom for natural gas delivered in November and December.
"They finally paid for October, but they are evidently not paying for November and December. This was made rather clear to us," Kupriyanov said.
The spokesman said the price of Russian-supplied gas for Ukraine next year could be reasonable, although the country's debt has hampered progress in talks.
"The price could be quite acceptable for Ukrainian consumers," he said, adding Ukraine's outstanding gas debt was the main problem at the moment.
Kupriyanov said Gazprom has not yet started preparations to cut gas supplies to Ukraine over Kiev's outstanding debt.
"We have not yet started technical arrangements, but the zero hour will strike on January 1," he said.
