The European Energy Secretariat said it was seeking a decision on whether or not Serbia was breaking energy laws in the European Union.
"The Secretariat has repeatedly expressed concerns that Serbia’s two transmission system operators for natural gas, Srbijagas and Yugorosgas, do not comply with unbundling requirements," the regulator said in a statement.
Unbundling means companies are prohibited from controlling both production and distribution of natural gas.
The Energy Community Secretariat issued a similar complaint in February, which followed a meeting between Srbijagas and Russian energy company Gazprom to discuss the South Stream natural gas pipeline, a transit system that would run through the Black Sea to European consumers.
The secretariat said Wednesday it was frustrated with Serbia's "lack of an adequate response" and "the extensive delay" regarding its concerns. Under the February order, Serbia had two months to response to the secretariat's complaint.