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.
Source: Al Jazeera