LAUSANNE, Switzerland — With substantial progress made on some of the toughest elements of a final nuclear deal in recent weeks, Iran and the six world powers (UN Security Council plus Germany, or P5+1) find themselves struggling to resolve differences over two seemingly more peripheral issues at meetings here this week. They involve the scope of research and development on centrifuges that Iran can conduct during the period of a final nuclear accord, and Iran’s desire for the lifting of UN Security Council sanctions on its nuclear program that it sees as treating Iran as a pariah state.

The hold-up is likely to require negotiators to return next week to try to complete a political accord on major elements of a final nuclear accord that diplomats had hoped to complete this week, ahead of a self-imposed end of March deadline and the threat of new Congressional Iran sanctions.

Iranian Foreign Minister Mohammad Javad Zarif, speaking to Iranian journalists the morning of March 18 ahead of meetings with US Secretary of State John Kerry, said he did not anticipate other foreign ministers from the P5+1 joining the talks here at this lakeside Swiss city this week. The comments were seen as a signal that he anticipated negotiators were likely to have to continue work next week to complete a political accord, if remaining differences can be resolved.

"I don't think their presence will be needed in this round because when the solutions are found and we approach a deal, then all the foreign ministers of the negotiating parties should come," Iranian state media cited Zarif, AFP reported.