Updates throughout with quote from President Vucic, background, details

BELGRADE, June 16 (Reuters) - Hungary's MOL and Russia's Gazprom Neft and Gazprom should get a 15-day extension for talks on the purchase of a 51.56% stake in the Serbian-based oil firm NIS, Serbian President Aleksandar Vucic said on Tuesday, as an initial deadline expired.

The U.S. Treasury's Office of Foreign Assets Control imposed sanctions on NIS last October as part of its attempts to target Russia's energy sector over the war in Ukraine and demanded Russian firms divest the shares they own in NIS.

The U.S. sanctions also forced NIS, which owns the only refinery in Serbia, to seek waivers from OFAC to buy and process crude oil, which it would not require once the share sale is completed.

MOL signed an agreement in January to buy the combined Gazprom Neft and Gazprom 51.56% stake, pending OFAC approval. Washington gave them until June 16 to complete the sale.

Vucic said in a live broadcast from the Georgian capital Tbilisi where he is on a state visit, he had been informed there should be a 15-day extension.

He added he hoped the Russian side would accept the deal.

MOL, NIS and Gazprom Neft could not be immediately reached for comment.

Also on Tuesday, Serbia which holds 29.9% of NIS and MOL formally signed a shareholders' agreement over Belgrade's minority stake in the company, which is separate from acquisition talks between MOL and Russian companies.

Under the deal with MOL, the Serbian government will be able to buy an additional 5% stake in NIS, if Gazprom Neft and Gazprom agree to sell their stake and if OFAC approves the transaction.