BELGRADE (Reuters) -Serbia has ample crude oil and fuel in storage to meet demand in the short term but U.S. sanctions set to come into force on October 1 cast a shadow over the landlocked Balkan nation's future energy security, sources told Reuters on Friday. Russian-owned oil company NIS said earlier it had secured adequate crude and fuel stocks after Serbian President Aleksandar Vucic confirmed that the U.S. would not extend further a waiver for sanctions first announced in January. NIS did not disclo ...