Ukraine, Russia and the EU struck an agreement on Sunday that should enable the resumption of Russian supplies via Ukraine to Europe, large parts of which have been plunged into a mid-winter energy crisis.
But it was unlikely the gas would reach Europe earlier than Tuesday, nearly two weeks after a gas price row broke out between Moscow and Kiev, choking off supplies and raising new questions about the EU's reliance on Russian energy.
The agreement signed on Sunday is for international teams of monitors to deploy to pumping stations along the route of gas pipelines through Russia and Ukraine to Europe -- a condition set by Moscow to start gas flowing again.
Russian gas export monopoly Gazprom said in a statement the firm was waiting to receive a faxed copy of the agreement, signed in the early hours of Sunday in the Ukrainian capital, before letting monitors start work.
Once they are in place, the gas taps will be re-opened but it is likely to be a further 36 hours before the fuel reaches customers in Europe because of the time it will take for pressure to build up in the pipeline network.
"Monitors are ready to fly to gas compressor stations in Russia, Ukraine and Europe. They can get there today, which means gas can resume flowing today as well," a Gazprom official told Reuters.
Europe receives 80 percent of its Russian gas -- or a fifth of its total needs -- from pipelines that run via Ukraine.
The gas disruptions have led many European countries to question whether Russia and Ukraine, former Soviet neighbours which have clashed repeatedly over gas and Kiev's drive to join the NATO alliance, are reliable energy partners.
The presence of the monitors at pumping stations is designed to reassure Russia that all the gas it sends via Ukraine is reaching customers in Europe.
Moscow had earlier accused Kiev of siphoning off gas intended for Europe, a charge Ukraine denied.
A Reuters cameraman travelling with a group of five EU monitors said they had landed in the eastern Ukrainian city of Luhansk on their way to a compressor station near the Russian-Ukrainian border.
Even once gas starts flowing again to Europe, Ukraine will still be without Russian gas for its own use. The latest talks on a new contract ended without result on Saturday.
Anger at disruptions
Moscow and Kiev have blamed each other for the disruptions, which began on New Year's Day when Gazprom cut off all supplies for Ukraine's own use after failing to reach agreement on a new gas supply contract.
Since then, a total of 18 countries have suffered problems with their gas supplies..
Eastern and central Europe have borne the brunt of the supply disruptions, with Bulgaria shutting schools because it could not heat them and Slovakia saying it would re-start a nuclear reactor which it shut down last year.
Heating has returned to hundreds of thousands of homes in the Balkans since Friday as energy companies switched to alternative fuels and started importing gas from elsewhere.
Temperatures overnight fell to 17 degrees Celsius below zero in some parts of the Balkans. In Bulgaria, the EU's poorest country, many people were still using electric heaters to compensate for lower-than-normal gas heating.
Sofia residents expressed anger at the hardship they had suffered.
"Half of Europe has become a hostage of the squabbling between Russia and Ukraine. This is pure blackmail, totally unacceptable and we should demand financial compensations," said Krasimira Dimitrova, 56.
The breakthrough in the row came after Czech Prime Minister Mirek Topolanek, whose country holds the EU presidency for six months, flew first to Moscow and then to Kiev to persuade their governments to sign the monitoring agreement.
"We were able to reach a political agreement aimed at getting out of the deadlock ... Ukraine has accepted all the terms needed for Russia to supply gas," Topolanek said in Kiev.
Ukraine says it has enough gas stockpiled to last until the end of the winter, though the government has restricted supplies to industrial customers.
Russia says it has been subsidising fuel supplies to Ukraine for years and now wants it to pay $450 per 1,000 cubic metres of gas. That is roughly in line with the price EU customers pay but a huge increase on the $179.5 Kiev paid last year.