Ukraine must make a down-payment to Russia for gas and in return Moscow must offer a fair price for further deliveries, Europe's Energy Commissioner Guenther Oettinger said ahead of talks with the Russian and Ukrainian energy ministers in Berlin on Friday.

Oettinger is acting as mediator in the talks, which on Monday saw the two governments agree to study his proposal that Ukraine pay Russia $2 billion by Thursday and a further $500 million by June 7.