Serbia has handed control of its energy supplies to Russia in a deal signed by the governments of both countries.
Serbia's energy supplies are now under Russian control
Under the agreement, Russia's state-owned gas giant Gazprom will buy 51% of Belgrade's NIS oil monopoly and run a gas pipeline through Serbia.
The financial terms have not been disclosed, but the deal could be worth as much as $2bn (£1bn).
The European Union has suggested that the agreement could be politically motivated, but Serbia has denied this.
The deal comes as Gazprom prepares to build a 900-km (550-mile) gas pipeline under the Black Sea from Russia to Bulgaria, with Serbia's backing considered vital to the success of the project.
"Serbia is becoming a key hub in the prospective Russian energy supply to southern Europe," Russia's President Vladimir Putin said.
The deal comes in the midst of a presidential election campaign and amid wrangling over the future of Kosovo, technically still a southern province of Serbia.
Russia has backed Serbia's continued control of the region, while the EU has supported moves for Kosovo to declare its independence.