Ukraine's talks with the European Union on the possibility of importing gas from the West to substitute for Russian supplies raises questions about the legality of such a move, the head of Russia's gas monopoly Gazprom told Rossiya 24 television.
Ukraine said on Friday it was urgently seeking ways to import natural gas from the West after Moscow almost doubled its discounted gas tariff for Kiev last week. Ukraine covers half its needs with Russian gas.
One possibility discussed with the EU is "reverse flows", in which EU countries, possibly Slovakia, would send gas back down pipelines normally used to carry Russian supplies through Ukraine to the West.
"When it comes to reverse supplies - well, this raises several questions," Alexei Miller, Gazprom's chief executive officer said in an interview aired on Saturday.
He said that reverse supplies from Slovakia may not be physically possible, which could mean it would be a "virtual reverse just on paper".
"This issue requires a very careful study and consideration," Miller said.