This undated photo posted on the website of Russian energy giant Gazprom shows Nord Stream 2 pipe-laying operations in German territorial waters. (PHOTO / XINHUA)

The 1,230-km pipeline is expected to bring 55 billion cubic meters of natural gas annually from Russia to Germany.

MOSCOW - The gas-in procedure for the first string of the Nord Stream 2 natural gas pipeline has started, the project's operating company said on Monday.

"This string will be gradually filled to build the required inventory and pressure as a prerequisite for the later technical tests," Nord Stream 2 AG said in a statement.

The first string of the pipeline has undergone pre-commissioning activities to assure the pipeline integrity, while pre-commissioning steps for the second string are underway, it added.

Russia's gas industry giant Gazprom, which owns Nord Stream 2 AG, announced on Sept 10 the completion of construction of the flagship energy project, although the United States has voiced opposition repeatedly.

The 1,230-km pipeline is expected to bring 55 billion cubic meters of natural gas annually from Russia to Germany via the Baltic Sea. Gazprom intends to put it into operation before the end of the year.

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