Nord Stream is part of a network of pipelines planned to diversify Russian natural gas exports to European consumers. The first leg of the pipeline system went into service last year.
Vitaly Markelov, deputy chief executive officer at Gazprom, was quoted by the Platts news service as saying the first line was delivering natural gas at a rate of 971 billion cubic feet per year.
"Yesterday, we reached the maximum capacity of Nord Stream," he said.
The three sections that make up the second offshore leg are set for pressure testing and connection this summer. They'll be commissioned this year as part of an automated twin pipeline system.
The lines, once fully operational, will transport around 1.9 trillion cubic feet of Russian gas each year to European consumers for at least 50 years.
In early May, the consortium behind the project said it would spend the rest of the year examining the feasibility of adding up to two additional pipelines to the network.