The source, however, said that Gazprom, which leads the project together with Italy's ENI, is not considering reducing its stake.
Earlier on Friday, ENI's CEO, Paolo Scaroni, said in an interview with the Wall Street Journal that a German company would be joining South Stream in the next few days, which partners hope will help the pipeline gain favour with the European Union.
Scaroni said South Stream will only work if it is exempted from the EU's unbundling rules under the Third Energy Package - a prospect that he saw as being bolstered by Germany's signing up.
"We need to have the EU backing the project, and having three companies from three major countries of the EU as part of the project should be a positive," he said.
Gazprom could not be reached for comment.
France's EDF also holds a stake in the pipeline that will transport up to 63 billion cubic metres of gas to Central and Southern Europe. The cost is estimated at around $21.5 billion.
Wintershall has long been rumoured to become South Stream's fourth stakeholder, with ENI saying last year that the German company was considering joining. Wintershall is already a partner in another big Gazprom-led project, Nord Stream — a pipeline which will transport Russian gas across the Baltic Sea into Germany.
South Stream is looking to gain more favour in the EU, which already supports the rival $10.8 billion Nabucco project that will sidestep Russia and Ukraine by piping gas directly from Central Asia to Europe under the Black Sea.
(EurActiv with Reuters.)
