The European Union on Tuesday asked Bulgaria to suspend work on the multinational South Stream pipeline project designed to allow Russia's gas deliveries to bypass Ukraine on their way to Europe.
The European Commission said the planned pipeline under the Black Sea may break EU rules.
South Stream is one of Russia's major projects to reduce its reliance on Ukraine as a transit country after years of price disputes and the crisis over Crimea.
"We sent (Bulgaria) a letter of formal notice -- we formally asked for information and while discussions are taking place ... we also asked them to suspend the project," said European Commission spokesman Antoine Colombani.
The European Parliament in mid-April urged that the project, led by Russian gas giant Gazprom, be scrapped as a rebuke to the Kremlin for its actions in Ukraine.
But the Commission, the EU's executive, has so far refrained from pushing for an outright cancellation and has instead engaged in a legal showdown with Gazprom and Bulgaria over regulatory details of the project.
The Commission says the project breaks EU rules that stop gas suppliers from also controlling access to supply networks.
It also said that tendering for work on the project may not have been carried out in accordance with EU rules.
Bulgaria depends almost entirely on Russian gas deliveries and has long backed South Stream as a way to guarantee security of its supplies, meaning that the country has objected to EU resistance to the deal.
In an interview Sunday, EU Energy Commissioner Guenther Oettinger said South Stream would remain at a standstill without a change in attitude by Russia over Ukraine.
"We will restart renegotiations when our Russian partner once again conforms with international law and is ready for constructive cooperation on the project and takes our energy rights into account," he told the newspaper Frankfurter Allgemeine Sonntagszeitung.
The South Stream project, if completed, would also pass through Serbia and the EU nations of Greece, Italy, Hungary, Slovenia and Austria.