"South Stream is likely to compete with Nabucco in the long term," Oettinger told journalists in Brussels, during a conference marking the 10th anniversary of the EU-Russia Energy Dialogue.
The European Commission is seeking to diversify energy supplies to Europe, he said, adding that gas imports are expected to increase in the next few years.
The South Stream project, in which Russia, Bulgaria and Greece are partners, stipulates the construction of a 300-kilometer pipeline from the Bulgarian Black Sea port of Burgas to Alexandroupolis on the Greek Aegean Sea coast. The pipeline is promoted by the Russian authorities as a rival to the Nabucco project.
Russia's gas giant Gazprom CEO Alexei Miller said earlier in the month that the pipeline may be put into operation in August 2015.
