Herzegovina ,September 04 , SPA -- A Bosnian oil company is looking for international partners to explore domestic oil fields that may contain more than 100 million barrels, officials said Thursday, AP reported. Bosnia's biggest energy company, state-run Energoinvest, invited foreign companies to explore five already identified sites over the next five years. Companies willing to invest at least US$100 million can send nonbinding letters of interest by Oct. 3, Energoinvest marketing director Ibrahim Bosto said, adding that a tender will be issued by the end of the year. British and U.S. companies, together with Energoinvest, have already explored here decades ago and defined five potential sites where it would be worth drilling. Some 1,000 tons were pumped out in 1991, but the 1992-95 war disrupted the project. Energoinvest believes there are up to 100 million barrels, according to the results of prewar explorations with British and U.S. companies, Bosto said.