Russia has made a last-minute attempt to put pressure on Ukraine over signing a landmark EU trade treaty.
Russian Prime Minister Dmitry Medvedev said it will ask Ukraine to start pre-paying for the vital gas supplies it receives.
Ukraine as it hopes to sign a landmark association agreement with the European Union next month, which would establish a free-trade zone and bolster political ties.
Russia has warned it that doing business between the two countries will be more difficult if Ukraine signs the deal.
Mr Medvedev was reacting to a complaint by Alexei Miller, chief executive of Russian gas giant Gazprom, who said Ukraine owes Russia $882m for the August deliveries and was due to pay for it by October 1.