A senior U.S. administration official said on Wednesday it would be "very hard", though still possible, for six world powers to reach an initial nuclear deal with Iran in talks in Geneva this week.
"I think we can (get a deal), whether we will, we will have to see because it is hard. It is very hard," the official told reporters after talks between the six and Iran resumed in the Swiss city.
The official also said that the vast majority of sanctions on Iran would remain in place after any preliminary accord on limiting its disputed nuclear programme, and that Washington would "vigorously" implement them.
The five permanent U.N. Security Council members plus Germany are meeting with Iran to negotiate a deal that would lead to the freezing of some sensitive parts of Tehran's nuclear program in exchange for temporary relief from international sanctions that have crippled the Iranian economy.
It is the third round of Iran negotiations in Geneva since October.
"These negotiations are difficult," the official said. "They are tough. There are moments of tension. There are moments of even humour, occasionally."
Responding to U.S. and Israeli critics of the proposed sanctions relief for the Islamic Republic, the official said the proposed relief on offer to Tehran is "quite small."
The proposed measures would include allowing Iran access to some frozen funds in overseas accounts in instalments, the ability to trade in some precious metals and a possible temporary lifting of pressure on countries not to import Iranian oil.
The official said that it would be best for public figures in Iran and the United States to refrain from rhetoric that deepens the mistrust between the two estranged nations, which have not had diplomatic relations since 1980.