US Secretary of State John Kerry was set on Saturday to try and seal a historic deal with Iran that would curb its nuclear programme in exchange for relief from painful sanctions.
But just days ahead of Tuesday's deadline for Iran and six major powers to nail down the accord they hope will end a 13-year standoff, diplomats on both sides said Friday that major differences remain.
As a result, the June 30 target date may slip -- if only for a few days -- setting the stage after almost two years of hard bargaining for yet another bruising and lengthy round of talks.
"Some major problems exist which are still blocking the work... but in other areas we have made good progress," Iranian Deputy Foreign Minister Abbas Araghchi told state television Friday from Vienna, where the talks will be held.
"Overall, the work is moving ahead slowly and with difficulty," he added.
This was echoed by a Western diplomat, who said that several key issues including a stalled UN probe into Iran's past activities and the timing of sanctions relief remain "extremely problematic".
"The most difficult issues need to be resolved in the coming days: transparency, inspections, PMD (possible military dimensions of Iran's nuclear programme), sanctions... On the major issues there is major disagreement," the diplomat said.
A senior State Department official said Kerry would meet Iranian Foreign Minister Mohammad Javad Zarif on Saturday morning "to discuss the ongoing P5+1 nuclear negotiations".
A deal, it is hoped, would put an end to a crisis dating back to 2002 that has threatened to escalate into war and has poisoned the Islamic republic's relations with the outside world.
Even if negotiators manage a deal, it will be closely scrutinised by hardliners both in Iran and the United States, as well as Iran's regional rivals Israel, widely assumed to have nuclear weapons itself, and Saudi Arabia.
In return for downsizing its activities and allowing closer UN inspections, Iran, which denies wanting nuclear weapons, would see the progressive lifting of UN and Western sanctions that have choked its economy.
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- Shifting red lines? -
In what was hailed as a massive breakthrough, the P5+1 and Iran in April reached an accord in Lausanne, Switzerland, on the main outlines of a deal, aiming to finalise it by the end of this month.
These include Iran slashing by more than two-thirds the number of uranium enrichment centrifuges, which can make fuel for nuclear power or the core of a nuclear bomb, and shrinking its uranium stockpile by 98 percent.
Iran also agreed to change the design of a planned reactor at Arak so that it cannot produce weapons-grade plutonium and to no longer use its Fordo facility -- built into a mountain to protect it from attack -- for uranium enrichment.
But on Tuesday Iran's supreme leader, Ayatollah Ali Khamenei, set out key "red lines" for the final agreement that appeared to go against parts of what was agreed in Lausanne.
Khamenei said that all economic and financial sanctions imposed by the UN Security Council and the US must be lifted on the same day that a final agreement is signed.
Western powers have said no sanctions will be eased until the UN atomic watchdog has verified that Iran has taken key steps agreed to under the deal, including cooperating with the UN's probe into past activities.
Khamenei also took issue with the IAEA visiting military sites -- vital for the UN investigation -- and with the time periods for which Iran will halt certain activities.
Arms Control Association analyst Kelsey Davenport said that although the comments were "unhelpful, they are unlikely to derail the talks".
"(It) is important to remember that Khamenei's 'red lines' have shifted over time and he has made statements in the past that were not always reflected in the outcomes of the negotiations," she said.
While Kerry will meet his Iranian counterpart on Saturday, France's foreign minister Laurent Fabius -- seen as a hardliner in the talks -- was also expected in Vienna along with other foreign ministers.
"We are going to have some tense and complicated days and nights. We all need to stay calm and keep our cool," the Western diplomat said.