Ukraine's energy minister, Yuriy Boiko, has said Ukraine is receiving less natural gas from Russia than required under current contracts.
The remark comes one day after the European Union said flows to six European states had returned to normal after deliveries fell when temperatures plummeted on the continent.
But Boiko said that Russian natural-gas monopoly Gazprom is shipping 450 million cubic meters per day instead of the stipulated 510 million.
Most of the gas Russia ships to Ukraine is bound for countries in Western and Central Europe.
Boiko attributed the shortfall to severely cold weather across Russia.
Earlier this week, Gazprom said it could not pump additional gas to European countries.
European Union officials said Russian gas deliveries to nine European countries have dropped since the cold weather set in.
Compiled from agency reports