An adviser to Iran's Supreme Leader Ayatollah Ali Khamenei lashed out at world powers amid negotiations to reach a preliminary nuclear accord in Switzerland Monday.
"Our negotiating team are trustworthy and compassionate officials that are working hard, but they should be careful with the enemies' deceptive and skillful tactics," the adviser, Ali Akbar Velayati, told Fars news agency.
For days Iran, the United States, Britain, France, Germany, Russia and China have been trying to break an impasse in negotiations aimed at stopping Tehran from having the capacity to develop a nuclear bomb, in exchange for an easing of United Nations sanctions that are crippling its economy.
But officials at the talks in the Swiss city of Lausanne said attempts to reach a framework accord, which is intended as a prelude to a comprehensive agreement by the end of June, could yet fall apart.
Negotiators from all parties appeared increasingly pessimistic. "If we don't have some type of framework agreement now, it will be difficult to explain why we would be able to have one by June 30," said a Western diplomat.
He said three major sticking points must be resolved if Iran and the six powers are to secure the deal before March 31, and it is unclear whether those gaps could be filled.
The diplomat said the most difficult issues related to the duration of any limits on Iranian uranium enrichment and research and development activities after an initial 10 years, the lifting of the sanctions and the restoring of them in case of non-compliance by Iran.
"It seems that we have an accord for the first 10 years, but with regard to the Iranians the question of what happens after is complicated," the official said on condition of anonymity, adding: "I can't say what the final result will be."
German Foreign Minister Frank-Walter Steinmeier said there had been "some progress and some setbacks in the last hours".
Highlighting the general mood, a diplomat quoted by Chinese news agency Xinhua said the atmosphere on Monday had turned from "optimism" to "gloom" among negotiators.
In addition to US Secretary of State John Kerry, Iranian Foreign Minister Mohammad Javad Zarif, Steinmeier, British Foreign Secretary Philip Hammond, French Foreign Minister Laurent Fabius, China's Wang Yi and Russia's Sergei Lavrov gathered at a 19th-century hotel overlooking Lake Geneva.
After the first meeting since November of all the ministers, Lavrov returned to Moscow for an engagement, though officials said he would return if there was something to announce.
Prime Minister Benjamin Netanyahu, who has campaigned against the negotiations, said in Jerusalem that the agreement being put together in Lausanne sends the message "that Iran stands to gain by its aggression."
Western officials said the two sides had previously been closing in on a preliminary deal that could be summarized in a brief document which may or may not be released.
Officials said the talks were now likely to run until the deadline of midnight on Tuesday or beyond.
The six powers want more than a 10-year suspension of Iran's most sensitive nuclear work. Tehran, which denies it is trying to develop a nuclear weapons capability, demands a swift end to sanctions in exchange for limits on its atomic activities.
Both Iran and the six have floated compromise proposals but agreement has remained elusive.
One sticking point concerns Iran's demand to continue with research into newer generations of advanced centrifuges. These can purify uranium faster and in greater quantities than those it currently operates for use in nuclear power plants or - if very highly enriched - in weapons.
Another question involves the speed of removing the sanctions on Iran.
Even if a framework deal is reached by the deadline, officials say it could still fall apart when the two sides attempt to agree on all the technical details for the comprehensive accord by the end of June.
There were several examples of progress and setbacks. Western officials said Iran suggested it would could keep fewer than 6,000 centrifuges in operation, down from its current figure of nearly 10,000, and ship most of its enriched uranium to Russia.
But Iranian negotiator Abbas Araqchi said dispatching stockpiles abroad "was not on Iran's agenda".
A senior US State Department official said there had been no decisions on stockpiles, though several officials made clear that the Iranians had given preliminary consent to the idea before reversing their position. Still, negotiators said stockpiles were not a dealbreaker.
It was not clear if the Iranian backtracking on certain proposals was a sign that Tehran might be getting cold feet.
On the issue of UN sanctions, officials expressed concerns that the five permanent veto-wielding members of the UN Security Council could object to plans to strip away some of the UN measures in place since 2006, albeit for different reasons.
Britain, France and the United States want any removal of UN sanctions to be automatically reversible, but the Russians dislike this because it would weaken their veto power, a Western official said.