Chinese workers have completed a tunnel beneath a border river with Russia, the most significant part of the crude oil pipeline project linking the two countries.

More than 80 workers had been drilling the 1,090-meter tunnel beneath the Heilong River since Sept. 1, 2009, said Li Changcai, director of the construction team of China National Petroleum Corporation (CNPC).

The tunnel, considered the most difficult part of the entire pipeline project, had taken experts from both sides about two years to design and verify its blueprint.

The remaining work -- installing the pipeline through the tunnel -- would be relatively easy and was expected to be finished by the end of the month, Li said.

The 970-km pipeline would carry oil from eastern Siberia to the Chinese city of Daqing, in Heilongjiang Province, and have a capacity of 15 million tonnes per year.

Construction of the pipeline was expected to be completed in October, said Li.

Chinese Foreign minister Yang Jiechi said earlier this month that the project would become operational in 2011.

Most Russian crude oil imported via the pipeline will be refined in the Daqing and Fushun refineries, owned by subsidiaries of CNPC, the parent company of PetroChina.

Under an oil-for-credit agreement signed in February 2009, China will grant Russian oil firms loans of 25 billion U.S. dollars for a term of 20 years in exchange of 15 million tons of oil deliveries annually over the next two decades.