Russia has accused Ukraine of stealing gas meant for Europe
Russian state energy company Gazprom has resumed some gas supplies through Ukraine to Europe, after they were cut for nearly a week.
Russian TV stations showed a Gazprom official ordering a resumption of supplies to Balkan countries.
Analysts say supplies could in theory return to normal within 24 hours, but a more likely time frame is 36-48 hours.
Hundreds of thousands of Europeans were left without gas when Russia turned off the taps in a dispute with Ukraine.
The BBC's Rupert Wingfield-Hayes, at the Sudzha pumping station on the Russian-Ukrainian border, says the underlying cause of the crisis has not even started to be resolved, and Russia is still not providing Kiev with any gas for its domestic consumption.
The two countries are locked in an ongoing bilateral dispute over debts and prices.
Rupert Wingfield-Hayes reports from a gas pumping station in Sudzha
Disagreement remains over how much Ukraine should pay Russia for its gas, and what Russia should pay Ukraine in return for transporting gas to Europe.
The technician responded: "I have received the order. We are implementing it."
Our correspondent says that, soon after the announcement, gauges started to show that gas was flowing.
There has been no word on whether gas supplies through four other pumping stations into Ukraine have been resumed.
Ukrainian officials said the gas could take around 36 hours to reach Europe.
But Gazprom spokesman Sergey Kupriyanov said the pressure in the pipelines should be sufficient for the gas to be pumped in and out of Ukraine simultaneously.
"As soon as we start pumping the gas at the entry point, it should appear at the exit point," he said in a statement.
The European Commission reacted positively to the news that gas was flowing again.
"The commission welcomes the announcement from Russia that the gas flow is back in the pipes," spokesman Ferran Tarradellas told AFP news agency.
"Our monitors are checking this on the ground."
Under the deal agreed on Monday, EU and Russian observers will monitor supplies in and out of Ukraine, in order to calm Russian fears that Ukraine is siphoning off gas for its own use. Ukraine has denied this allegation.
Despite a deal signed by Russia, Ukraine and the Czech presidency of the EU, it may be some time before supplies return to normal.
Russia cut gas supplies to Ukraine on New Year's Day, saying it would pump only enough for customers further down the pipeline.
Thousands of Europeans have been left without gas in freezing temperatures
But then Moscow accused Ukraine of siphoning off gas intended for third countries and it restricted supplies even further.
Ukraine denied the claim, but the flow of Russian gas ceased completely on 7 January, leaving many European countries with major shortages.
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.
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