Russia's Gazprom will not postpone the timeline of transferring gas supplies to Ukraine to prepayment anymore, Gazprom CEO Alexei Miller said.

"Russia met the European Commission halfway several times and postponed the timeline of transferring Ukraine to prepayment. This timeline will not be moved anymore," he said.

"By 10 a.m. [Moscow time] on June 16 Gazprom is to see $1.951 billion on its account, otherwise prepayment will be introduced," Miller said.

This sum includes the debt for gas supplies in November and December 2013 - $1.451 billion - and Ukraine must confirm its commitment to pay debts for April and May "with an additional payment of $500 million," he said.