KIEV, May 27 - Ukraine raised the stakes in a dispute with Russia over gas supplies on Tuesday, saying Russian state-controlled company Gazprom owed Kiev natural gas worth around $1 billion which it had "stolen" when Moscow annexed Crimea, Reuters reported.

Russia has warned it will reduce gas supplies to Ukraine on June 3 if Kiev fails to pay in advance for next month's deliveries, causing concerns that onward supplies to Europe could be threatened.