Gazprom has reaffirmed its Monday deadline for Ukraine to pay its debts after talks between Moscow and Kiev ended without agreement.
The Russian gas giant said it will cut Ukraine's supply if Kiev fails to pay $1.95bn (£1.15bn) by 06:00 GMT.
"We reached no agreement. The chances that we meet again are slim," Gazprom spokesman Sergei Kuprianov said.
But EU Energy Commissioner Guenther Oettinger, who attended the talks, said he was "not pessimistic" about a deal.
He said he would continue to work for an agreement despite his compromise proposal, that Kiev pay $1bn on Monday and the rest in instalments, being rejected by Gazprom.
Ukraine's discounted rate for gas was axed in April after Moscow accused Kiev of failing to pays its bills.
Mr Kuprianov said Russia would switch to a pre-payment system if it did not receive the money, meaning Moscow could cut off gas supplies to Ukraine.
Read more: http://www.bbc.co.uk/news/world-europe-27862849