Speaking in Brussels, Piebalgs said: 'We haven't received any indication that there has been such a diversion.'
'Gas in Ukraine remains above envisaged volumes, which prevents us from fully meeting our obligations to foreign customers,' Medvedev said.
The Ukrainian government initially denied the accusations, but said that Ukraine's daily gas consumption had risen to a record 407 mln cubic metres on Sunday amid the severe cold snap, from 285 mln.
An official at Ukraine's state-owned Naftogaz energy firm confirmed to Agence France-Presse that gas supplies had been withheld on Sunday, and said that out of its overall monthly total, 'Ukraine had withheld exactly the amount agreed with Gazprom.'
'I don't know why Medvedev made these declarations. There was an agreement,' the official said.
Russia and Ukraine were involved in a bitter pricing dispute over gas delivered by Gazprom, which cut off supplies to Russia's neighbour on January 1 after negotiations on the issue failed.
The European Commission intervened and contributed to a resolution of the stand-off, as much of the gas bound for Europe transits through Ukraine.
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