Construction launched on 3,968 kilometre 'Power of Siberia' gas pipeline.
Dignitaries gather on Chinese side of the pipeline to witness the formal welding of the first joint in $55billion line.
Altai route for gas to China could take priority, reported Reuters.
Beijing reviewing its energy needs with slowing economy, says report.
Agreement means Power of Siberia pipe could be delayed, it is claimed.
Gazprom says operations are 'in full swing' with work due to start soon on iconic pipeline to provide gas on eastern route.
After months of speculation, parliament and President rubber-stamp deal that will see gas sent to China.
Pipeline will go through pristine Altai Mountains in supply route seen as economically vital by Kremlin.
New rail bridge linking the two countries symbolizes closest-ever economic co-operation between neighbors.