The first few meters of the pipeline - which when completed will be the world's longest at about 4,000 kilometers (2,500 miles) - were welded together at a ceremony in the Irkutsk Region town of Taishet, some 3,100 miles east of Moscow, the official said.
He added that 2,000 railway cars had delivered tube sections for the pipeline, being built by state pipeline monopolist Transneft.
Work on the pipeline's final route is still ongoing, after President Vladimir Putin said Wednesday that it should pass outside the drainage basin north of Lake Baikal, upsetting Transneft's plans to run a section just 800 meters from the lakeshore.
The plan had been at the center of controversy and protests by environmental groups, who said Lake Baikal, the world's largest body of fresh water and a Unesco-listed World Heritage Site, could suffer irreparable damage in the event of an accident on the pipeline.
