Bulgaria and Russia signed preliminary deals for the South Stream natural gas pipeline in 2010. Last year, Bulgaria received more than 95 billion cubic feet of natural gas from Gazprom, the pipeline's backer.
Bulgarian President Rosen Plevneliev said more transparency is needed in any deal with Gazprom regarding South Stream.
"Even I personally at this stage do not possess adequate information about what is to be signed," he was quoted by the Sofia News Agency as saying. The agency didn't report on what details may be lacking.
South Stream would divide into two pipelines -- one to Greece and the other through the Balkans -- after it passes through the Turkish waters of the Black Sea. The project would carry about 2.2 trillion cubic feet of gas per year when it goes into service by 2015.
In early November, the project company behind Nabucco, a rival gas pipeline backed by European leaders, announced the environmental impact assessment for the 256-mile section of the project through Bulgaria has entered the approval phase.