More EU sanctions against Russia 'not ruled out': Berlin
The ruling against the 44-year-old anti-corruption campaigner is "far off from constitutional principles," said Seibert, reiterating Merkel's call for Moscow to free him.
Nord Stream 2 is a 10-billion-euro ($11-billion) pipeline that will run beneath the Baltic Sea and is set to double Russian natural-gas shipments to Germany, Europe's largest economy.
It has long been in the crosshairs of the United States, particularly by the former Trump administration which promoted US gas and fiercely criticised European countries for their reliance on energy from Russia.
On Monday, France joined in a call urging Germany to scrap the pipeline, but Berlin said it was standing firm on the issue.