BELGRADE, Oct 16 (Reuters) - Russia will reduce gas supplies to Europe if Ukraine steals from the transit pipeline to cover its own needs, Russian President Vladimir Putin warned on Thursday, stressing he was "hopeful" it would not come to that.

Russia cut off supply to Ukraine on June 16 over what gas exporter Gazprom said were billions of euros in unpaid bills. Without Russian flows, there is concern Ukraine might have to siphon off gas from flows transiting the country en route to Europe this winter. (Reporting by Alexei Anishchuk; Writing by Matt Robinson; Editing by Alison Williams)