Adds comment by Gazprom Neft in paragraph 4

BELGRADE, May 6 (Reuters) - A recently formed Serbian company has offered 2 billion euros ($2.35 billion) to Russia's Gazprom Neft and Gazprom for their combined 56.1% stake in NIS NIIS.BEL, which operates Serbia's only oil refinery, its owner Ranko Mimovic said, adding that the bid had been "generally accepted" by the Russian owners of NIS.

The bid by KFT Senator Treasury G.T.7 Two LLC (Senator), which was founded last summer, represents a challenge to Hungary's MOL MOLB.BU, which is trying to buy a majority stake in NIS, which has been under U.S. sanctions since October last year.

The U.S. Treasury's Office of Foreign Assets Control imposed sanctions on NIS as part of wider measures targeting Russia's energy sector over the war in Ukraine. It demanded the divestment of the NIS shares owned by Gazprom Neft SIBN.MM and Gazprom GAZP.MM, its Russian majority owners.

Gazprom Neft said it was readying its stake sale in NIS to the Hungarian firm. "Gazprom Neft is actively preparing to sell its stake in NIS to the Hungarian company MOL. The necessary corporate and regulatory procedures are currently underway. The company is not conducting any other negotiations on this matter," it said in an emailed statement.

Gazprom and NIS did not immediately reply to emailed inquiries from Reuters.

Senator said it began proceedings before OFAC on October 30 to secure a license for the acquisition of a majority stake in NIS, and it informed Gazprom Neft about its plans on November 4. OFAC's website shows the request is "pending management review."

Gazprom holds an 11.3% stake in NIS, while Gazprom Neft owns 44.9%. The Serbian government owns 29.9%, with the remainder held by small shareholders and employees.

MOL signed an agreement on January 19 to buy the combined Gazprom Neft and Gazprom stake in NIS. OFAC gave the Russian companies until May 22 to complete the sale. Serbia also wants to increase its stake by an additional 5%.

Mimovic also said he informed the office of Serbia’s President Aleksandar Vucic about his plans on April 21. Vucic's office did not reply to a request for comment from Reuters.

Vucic said on January 26 that MOL was ready to pay 1 billion euros for a majority NIS stake.

($1 = 0.8503 euros)