Russia started pumping transit gas destined for European consumers into Ukraine on Tuesday for the first time since a contract row left Europe short of fuel in freezing temperatures.
A Reuters reporter at the Moscow headquarters of Russian gas export monopoly Gazprom said an official in the company's control room issued the order for pumping stations to start sending gas into Ukraine.
"The gas started moving at 10am (0700 GMT). Everything is fine," said the official.
The gas is expected to take between 24 and 36 hours to pass into waiting European pipelines, and with Moscow and Kiev still in dispute, there were differences over how Ukraine would ship gas to Europe while still being cut off itself.
Europe depends on Russia for a quarter of its gas supplies and most of those volumes are shipped though Ukraine, a former Soviet state whose pro-Western leaders have angered the Kremlin by seeking to join the NATO military alliance.
The disruption, which left European countries scrambling to find alternative energy sources and closed many factories, has damaged the reputation of Russia and Ukraine as reliable energy partners and prompted a search for new supply routes.
The resumption came after a European Union-brokered deal to deploy international monitors to strategic points along the pipeline route to reassure Russia that Ukraine was not siphoning off transit gas.