Bulgarian Prime Minister Plamen Oresharski said his government's role in the gas pipeline planned through southern European countries would have to wait for consent from a European Union wary of expanding Russia's energy influence.
Kremlin spokesman Dmitry Peskov said Monday any formal comment from Moscow would be premature.
"[The issue] needs to be clarified," he said. "It is early to say anything."
Russian energy company Gazprom said last month it signed the necessary contracts needed to start construction of the onshore section of the pipeline later this year. South Stream has an optimum capacity if 2.2 trillion cubic feet per year and should begin service by late 2015.
European lawmakers in April passed a resolution to reconsider the pipeline. Oresharski said following meetings with visiting U.S. senators he put the project on hold in Bulgaria at the request of the European Commission.
"Further proceedings will be decided after additional consultations with Brussels," he said.
The Serbian government said Monday it was following Bulgaria's lead and leaving the project behind.