"I cannot tell you today that our diplomacy will succeed because I am not sure that it will," Xinhua quoted Wendy Sherman, undersecretary of state for political affairs, as telling senators at a hearing.
"I can tell you that the past six months, we have made significant and steady progress," she said. "We've exchanged ideas, narrowed gaps on key issues, and identified areas where more hard work is required."
Under an interim deal reached between Iran and Britain, France, the US, Russia, China, and Germany that went into effect Jan 20, Iran would suspend some sensitive nuclear activities in exchange for a limited sanction relief, and the two sides will negotiate over a comprehensive deal within six months.
Iran and the six world powers agreed July 19 to extend their talks until Nov 24, as disagreements remained over Tehran's uranium enrichment capacity, its Arak heavy water reactor and the sanctions following a 16-day negotiations in Vienna.
"We will use this time to continue working towards that comprehensive plan for ensuring that Iran does not obtain a nuclear weapon, and that its program is exclusively peaceful," Sherman said at the Senate Committee on Foreign Relations hearing.
But she declined to say whether the talks would be extended again beyond Nov 24.