Russian President Vladimir Putin and Chinese leader Xi Jinping on Monday launched the first gas pipeline linking the two countries.
The mammoth Power of Siberia pipeline connecting the world's top gas exporter and its largest energy importer crowns years of tough negotiations and work in difficult conditions.
"Today is remarkable, a truly historic event not only for the global energy market, but first of all for us and for you, for Russia and China," Putin said during a televised ceremony featuring the two leaders.
Xi said the project "serves as a model of... mutually beneficial cooperation between our countries."
"The development of Sino-Russian ties is and will be a foreign policy priority for both our nations," Xi said on Russian television in translated remarks.
Alexei Miller, head of Russian gas giant Gazprom which championed the project, said nearly 10,000 people had worked to build the enormous pipeline.
During the ceremony Miller was shown ordering workers to open a valve allowing gas to pass across the border into China.
"Gas is going to the pipeline system of the Chinese People's Republic," he said.
The 3,000-kilometre (1,850-mile) pipeline runs from remote regions of eastern Siberia to Blagoveshchensk on the border, then into China.
Russia and China signed a 30-year, $400 billion deal to build and operate the pipeline in 2014, after a decade of difficult talks. It was Gazprom's biggest contract.
The company is to supply China with 38 billion cubic metres (1.3 trillion cubic feet) of gas annually when the pipeline is fully operational in 2025.
Gazprom has stressed that the pipeline ran through "swampy, mountainous, seismically active, permafrost and rocky areas with extreme environmental conditions".
Russia is also planning to soon launch two more gas pipelines that will ramp up supplies to Europe while bypassing Ukraine.
TurkStream, which Putin and Turkish leader Recep Tayyip Erdogan hope to launch in January, is to transport Russian gas to Turkey.
Nord Stream-2, which would double Russian gas volumes to Germany, is expected to go online in mid-2020.