Ukraine illegally tapped 50 million cubic metres of Russian natural gas bound for European consumers, a spokesman for Russian energy giant Gazprom said on Sunday.
"Over the past 24 hours (since Saturday), we fed 295 million cubic metres of gas into the Ukrainian gas transportation system that was more than requested by European consumers, but we had only 270 million on the output (data)," Sergei Kupriyanov said.
Russia cut off gas supplies to Ukraine Thursday after last-ditch talks on Kiev's outstanding $2 billion gas debt and a new contract for 2009 failed late on New Year's Eve.
Some European countries, including Poland, Hungary, Bulgaria and Romania, have said that Russian gas deliveries through the Ukrainian pipeline have fallen.
The official said Gazprom has proposed to the European Union (EU) to conduct independent monitoring of gas transit via Ukraine.