BEIJING - The China National Petroleum Corporation said Wednesday that it had sped up laying natural gas pipelines connecting China and Russia, the largest energy cooperation between the two countries.

Welding of the northern part of the 3,371-kilometer east-route natural gas has begun, the company announced.

The east-route natural gas pipeline originates in northeast China's Heilongjiang Province and terminates in Shanghai in the east. Construction began in June 2015 and will be completed in 2020.

Upon completion, the pipeline will provide China with 38 billion cubic meters of natural gas from Russia annually. It will have far-reaching significance for optimizing China's energy structure, cutting emissions and improving air quality.

The Russian part of the pipeline began construction in eastern Siberia in 2014.

The rise of natural gas use is the result of the government promoting it as a cleaner alternative to coal.

China aims to raise the use of natural gas to 10 percent of the country's energy mix by 2020 and 15 percent by 2030, according to the National Development and Reform Commission.

China's demand for natural gas will continue to soar toward 2040, outstripping domestic output by around 43 percent, according to an International Energy Agency report published Tuesday.