UNITED NATIONS - Negotiations between Iran and six world powers on implementing a landmark November deal to freeze parts of Tehran's nuclear program in exchange for easing some sanctions have run into problems over the issue of centrifuge research, diplomats said.

The dispute over centrifuges highlighted the huge challenges facing Iran and the six powers in negotiating the precise terms of the Nov. 24 interim agreement. If they succeed, they plan to start talks on a long-term deal to resolve a more than decade-long dispute over Tehran's nuclear ambitions.

Among the issues to be resolved in political discussions due to begin in Geneva later this week is that of research and development of a new model of advanced nuclear centrifuge that Iran says it has installed, diplomats said on condition of anonymity.

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