Bulgarians protest about the gas crisis outside Ukraine's embassy in Sofia
Russian, Ukrainian and European Union officials have signed a deal which should pave the way for the re-opening of gas supplies to Europe.
The deal was signed by Ukrainian PM Yulia Tymoshenko, Russian PM Vladimir Putin and Czech PM Mirek Topolanek.
The deal is aimed at enabling a resumption of Russian gas deliveries to European countries through Ukraine.
Hundreds of thousands of European homes have no heating after gas shipments via Ukraine were halted on Wednesday.
Although both Ukraine and Russia had guaranteed that transit routes to Europe would be unaffected, mutual accusations between Kiev and Moscow resulted in supplies being cut.
EU monitors were expected to set out early from Kiev early on Sunday for the pumping and measuring stations on Ukraine's eastern and western borders.
If things go to plan and once those monitors are in place, says the BBC's Gabriel Gatehouse in Kiev, Russia should turn on the taps.
'Calming Russian fears'
Mr Topolanek, who has been in Ukraine for talks with Ms Tymoshenko, said the signing of the deal cleared the way for the resumption of Russian natural gas supplies to Europe.
"Nothing prevents Russia now from resuming gas supplies," he said.
Ukraine's PM met the EU envoy on Saturday evening
Russia's agreement came after five hours of talks between Russian Deputy Prime Minister Igor Sechin and officials from the European Union.
Under the deal, EU, Ukrainian and Russian observers will monitor supplies, in order to calm Russian fears that Ukraine is siphoning off gas for its own use. Ukraine has denied this allegation.
After signing the deal, Ms Tymoshenko said Ukraine's agreement signified its "goodwill" and role as an "honest transit country", the Associated Press reported.
Mr Putin is quoted on Interfax, the Russian news agency, as saying that the "transit of gas through Ukraine will start again as soon as the (transit) control mechanism starts to work".
Ukraine and Russia's bitter contractual dispute over gas prices and transit fees has affected several countries which are unlikely to have supply resumed before Monday at the earliest, despite the signing of this deal.
Normal service in affected countries such as Bulgaria, Slovakia, Bosnia-Hercegovina and Serbia will be delayed as it takes around 36 hours from the moment of turning on the taps for the gas to reach consumers, our correspondent adds.
But the underlying issue that caused this crisis - how much Ukraine should pay Russia for its gas, and what Russia should pay Ukraine in return for transporting gas to Europe - remains unresolved.
Talks on the sale of Russian gas to Ukraine ended without an agreement on Saturday.
Gazprom's offer price of $450 per 1,000 cubic metres was more than double the $201 Ukraine was prepared to accept, said Oleh Dubyna, chief executive of Ukrainian state energy company Naftogaz. Ukraine paid $179.5 in 2008.
Russia cut supplies to the Ukraine on New Year's Day.
"Unfortunately, the talks with Gazprom have finished with nothing," he said.
"The talks now have to proceed at a higher level."
Until this element gas dispute is resolved, our correspondent says, there is no guarantee there will not be a repeat of what has been Europe's worst energy crisis for years.
The EU gets a quarter of its gas supplies from Russia, 80% of which passes through Ukraine and more than 15 countries across central Europe have been hit by the shutdown of Russian supplies.
Serbia and Bosnia-Hercegovina are among the worst hit as many homes rely on heating stations that only run on gas.
Meanwhile, the Slovak government has decided to restart a nuclear power station because of the gas supply cuts, Prime Minister Robert Fico said.
Slovakia shut the Jaslovske Bohunice power station at the end of last year in order to comply with its EU accession agreement.