The European Union has told Bulgaria to suspend preparatory work on Gazprom's South Stream pipeline, which will bring Russian gas to Europe and bypass Ukraine.
The European Commission on June 3 said that Brussels was concerned that contracts for the project break the bloc's laws.
The commission has sent a letter of formal notice asking for information, a preliminary measure that could lead to infringement proceedings and possible fines for Bulgaria.
Bulgaria's energy minister said Bulgaria, and other countries through which South Stream will pass, should not be "hostages" to the crisis between Ukraine and Russia.
The 2,380-kilometer pipeline will pass under the Black Sea and then travel through Bulgaria, Serbia, Hungary, and Slovenia before arriving in Italy.
The EU is seeking to reduce its dependence on Russian energy in the wake of the Ukraine crisis.
Based on reporting by Reuters, AFP, and AP