LAUSANNE, Switzerland — Secretary of State John Kerry on Thursday said that negotiators were still grappling with difficult issues in the talks on limiting Iran’s nuclear program, but that they had made some headway.
“We are pushing some tough issues, but we made progress,” Mr. Kerry told reporters in Lausanne during a break in the talks.
It was not yet clear, Mr. Kerry said, whether the United States team would return to Washington on Friday or remain here through the weekend to continue negotiating with the Iranians.
Iranian officials have spoken optimistically about the chances for sealing an accord in the coming days. The Iranian statements may be intended to build public pressure on the United States and its allies to make concessions, as well as to strike a positive tone for the Iranian public.
But the United States and some of its European negotiating partners have spoken far more cautiously about the prospects for an accord in the near future.
A Western official, who asked not to be named because he was discussing private deliberations, said that unless Iran demonstrated significantly more flexibility, it was unlikely that an accord could be reached this week.
Officials have suggested in recent weeks that under an accord, Iran would be allowed to retain about 6,000 centrifuges for enriching uranium.
Among the key issues that have not been settled, Western officials said, are limits on research and development of new types of centrifuges that could enrich uranium far more efficiently than those the Iranians have now.
Another difficult question is how quickly to lift the economic sanctions that have been imposed on Iran by the United Nations Security Council.
The negotiators’ goal has been to achieve the outlines of an accord by the end of March, and then negotiate a detailed comprehensive agreement by the end of June.
Energy Secretary Ernest J. Moniz met Thursday morning with Ali Akbar Salehi, the head of the Atomic Energy Organization of Iran, to discuss an array of unresolved nuclear issues. That was followed by a three-way meeting among an American negotiating team led by Mr. Kerry; an Iranian delegation led by Mohammad Javad Zarif, the foreign minister; and Helga Schmid, a senior representative of the European Union.