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BELGRADE, June 10 (Reuters) - Serbia on Wednesday completed talks on a shareholders' agreement with MOL, Hungary's largest oil and gas corporation MOLB.BU, over its minority stake in the Russian-owned and U.S.-sanctioned NIS oil firm, Serbia's energy minister Dubravka Djedovic Handanovic said in a post on Instagram.

The relations between the Hungarian oil firm and the Serbian government, which holds 29.9% of NIS NIIS.BEL, are separate from acquisition talks between MOL, and NIS's current majority owners, Russia's Gazprom Neft SIBN.MM and Gazprom GAZP.MM, which hold 44.9% and 11.3%, respectively.

"Today, we successfully closed all open issues with the Hungarian MOL and reached a compromise regarding the shareholder agreement between the Republic of Serbia and MOL," Djedovic Handanovic said on Instagram.

The U.S. Treasury's Office of Foreign Assets Control imposed sanctions on NIS last October as part of wider measures targeting Russia's energy sector over the war in Ukraine, demanding the divestment of the Russian-owned shares in NIS, and forcing the company to seek temporary operating licences, which are allowing it to import and process crude oil.

MOL signed an agreement in January to buy the combined Gazprom Neft and Gazprom stakes, with Washington giving the Russian companies until June 16 to complete the sale.

Under the deal with MOL, Serbia will buy an additional 5% shares in NIS, if Gazpromneft sells MOL a 56.15% stake in NIS, and the OFAC approves the transaction.

"(This) will give the state additional rights when it comes to making or blocking decisions important for our country," Djedovic Handanovic said, adding that the Serbian members of the NIS Board of Directors will also have greater powers.

She also said that the Hungarian side pledged that the Serbia-based NIS refinery, the only one in the country, will continue to operate with at least the same average annual capacity as in the last four years before the introduction of sanctions.

The NIS refinery in the town of Pancevo, just outside Belgrade, has a maximum annual capacity of 4.8 million tonnes of crude oil.