Months of fighting between government forces and separatist rebels in eastern Ukraine has forced coal mines to cut production or close entirely, imperilling the country's electricity market.
Meanwhile Russia, which supplied about half of the gas Ukraine used last year, cut supplies on June 16 in a row over pricing and in the wake of Moscow's annexation of Crimea.
Asked in a televised interview if he thought Ukraine could survive without Russian gas, Prime Minister Yatseniuk said: "No ... The situation (in winter) will be extremely difficult."
Ukrainian state-owned gas company Naftogaz has put aside $3.1 billion to buy gas for the forthcoming winter period,Yatseniuk said on Friday.
Ukraine has also been trying to secure more gas from the European Union and cut consumption levels from last year's 50 billion cubic metres (bcm). It currently has 15 bcm in its gas reserves, Yatseniuk said.
The prime minister said Ukraine, a net exporter of thermal coal, used for power generation, is considering buying non-Ukrainian coal as domestic output has been hit by the conflict with infrastructure destroyed by artillery fire and supply networks disrupted.
Around half of the 115 coal mines in Ukraine, Europe's second-largest coal producer, have halted production and output fell 22 percent year-on-year in July to 5.6 million tonnes.
"The mines have been bombed so there's no production of thermal coal ... Without supplies to power plants, there are problems with electricity and heating," Yatseniuk said.
In the latest escalation of the conflict, Ukraine said on Friday Russia had launched a "direct invasion" of its territory after Moscow sent a convoy of aid trucks across the border.