BELGRADE (Reuters) -Talks between Belgrade and the United States over postponing sanctions against Russian-owned Serbian oil company NIS for a fifth time have been marred with difficulties, Serbian mining and energy minister Dubravka Djedovic Handanovic said on Monday. NIS has so far secured a fourth reprieve, which is due to expire on July 29, putting Serbia in a precarious position as it tries to balance its energy security against geopolitical pressures. "Negotiations are quite tough, because we are in an unenviable situation between two great powers, Russia and America," Djedovic Handanovic said in a live broadcast on Serbia's state RTS TV.