As a result of Europe’s resistance, Russia in December said it would shift the planned landing point for the undersea pipeline to Turkey. Russia and Turkey, though, have yet to reach a deal for that plan.

Speculation about extending the Turkey plan to include Greece as the gateway to Europe began heating up after Mr. Tsipras visited Moscow this month. Discussions were said to include a possible pipeline deal, as well as a discount on the price Greece pays to Gazprom for natural gas.

Greece gets about three-quarters of its gas from Russia, but for Gazprom, it is a relatively small customer. Even Turkey, despite being the second-largest market for Russian gas in the region after Germany, would not be a big enough buyer on its own for all the gas Gazprom would eventually plan to send through a Black Sea pipeline.

For a new pipeline to Turkey to make economic sense, the gas would need to flow through Greece to reach Central and Western European markets. But the Gazprom plan could encounter some of the same resistance from the European Union that led to the South Stream plans’ being scrapped last year.

On Tuesday, a spokeswoman for the European Commission said that any new project between Gazprom and Athens would need to pass European muster.

“We are ready to offer our assessment on any such agreement, any such deal, and its implications when we have something to base our assessment on,” Anna-Kaisa Itkonen, a spokeswoman on energy matters for the European Commission, told a daily news conference. But, she added, “Any pipeline or any other sorts of projects operating in the E.U. area must comply with E.U. legislation.”

The European Commission declined to comment on the expected antitrust case against Gazprom. But that case is expected to focus in part on whether Gazprom is blocking gas flows to some parts of Europe and thwarting customers’ efforts to diversify their supply.