Problems surrounding the construction of the South Stream gas pipeline are likely to top the agenda as Russian Foreign Minister Sergei Lavrov visits Bulgaria and Slovenia on July 7 and 8.
The pipeline would deliver gas from Russia under the Black Sea to Bulgaria, Serbia, Slovenia, Hungary, and Austria.
However, the project is facing delays over concerns in Brussels the pipeline could violate EU antimonopoly laws.
In Sofia on July 7, Lavrov held talks behind closed doors with Prime Minister Plamen Oresharski and also met President Rosen Plevneliev.
In an interview with local media, Lavrov said Bulgaria's EU membership should not have "a negative impact" on joint energy projects that would benefit both sides.
In Ljubljana on July 8, Lavrov is due to meet President Borut Pahor and acting Deputy Prime Minister and Foreign Minister Karl Erjavec.