Talks between Belgrade and the United States over postponing sanctions against Russian-owned Serbian oil company NIS for a fifth time have been marred with difficulties, Serbian mining and energy minister Dubravka Djedovic Handanovic said on Monday.

NIS has so far secured a fourth reprieve, which is due to expire on July 29, putting Serbia in a precarious position as it tries to balance its energy security against geopolitical pressures.

"Negotiations are quite tough, because we are in an unenviable situation between two great powers, Russia and America," Djedovic Handanovic said in a live broadcast on Serbia's state RTS TV.

NIS, majority-owned by Russia's Gazprom Neft and Gazprom, operates Serbia's only oil refinery covering much of its needs, and sanctions could jeopardise its crude supply. The facility has an annual capacity of 4.8 million tons.

The US Treasury's Office of Foreign Assets Control initially placed sanctions on Russia's oil sector on January 10, and gave Gazprom Neft 45 days to exit ownership of NIS.

The US Department of the Treasury did not immediately respond to a request for comment on the talks.

"We will be striving with all available means to continue the dialogue with both sides ... to protect our position," she said.

On February 26, Gazprom Neft transferred a stake of around 5.15 per cent in NIS to Gazprom in an attempt to ward off sanctions.

Gazprom Neft now owns 44.85 per cent of NIS, while Gazprom has 11.3 per cent. The Serbian government owns 29.87 per cent, with the remaining shares held by small shareholders.

NIS imports about 80 per cent of its oil needs through Croatia's pipeline operator Janaf. The remainder is covered by its own crude oil production in Serbia.