Officials from Europe, Ukraine and Russia have met in Poland to try and resolve a dispute over gas supplies and prices.
Moscow said Kiev now owes $3.5bn towards its gas bill.
Alexander Novak, Russia's energy minister, said that state-owned Gazprom might cut back on deliveries if Ukraine does not pay it's June bill in advance.
But at Friday's three-way talks in Warsaw, Ukraine again insisted that it would not pay the money to the Russian gas company and demanded international arbitration.
Al Jazeera's Nadim Baba reports from Moscow.