Beijing reviewing its energy needs with slowing economy, says report.
Altai route for gas to China could take priority, reported Reuters.
Agreement means Power of Siberia pipe could be delayed, it is claimed.
Construction launched on 3,968 kilometre 'Power of Siberia' gas pipeline.
Geologists say as much as 44.8 billion roubles-worth has been found in valley to the north of famous range.
New field will enable eventual exports to China via 'western route' through Altai Mountains.
Dignitaries gather on Chinese side of the pipeline to witness the formal welding of the first joint in $55billion line.
Pipeline will go through pristine Altai Mountains in supply route seen as economically vital by Kremlin.
Talks also underway on gas supply route to China via Altai Mountains.