Nuclear diplomacy with Iran is moving into unprecedented realms today, with unexpected prospects of striking a first-step deal in Geneva prompting US Secretary of State John Kerry to change travel plans and join the talks, along with British, French, and German foreign ministers.

That sudden development – after nearly two years of fruitless talks to limit Iran’s nuclear program in exchange for lifting sanctions – also prompted opponents as varied as Israel’s prime minister and Iran’s hard-line Friday prayer leader to raise loud voices of complaint.

Iran’s Foreign Minister Mohammad Javad Zarif told The Christian Science Monitor today that a joint text was being prepared, but that “a very difficult task” lay ahead.

“The only way that we can create confidence and sustain this process is to have balance in the first step, about each side trying to alleviate the most immediate concerns of the other side,” Mr. Zarif said in an interview as the day’s diplomacy began.

“We are still far from reaching an agreement, because we still have to nail them down and put them on paper, and in the best of circumstances we can finish it,” said Zarif.

US officials want an initial deal that would to “put time on the clock” by halting the advancement of Iran’s nuclear program in exchange for “very limited” sanctions relief until a final, far more sweeping deal is reached, perhaps within six months.

Iran wants the endgame defined to allow the “right” of a robust peaceful nuclear program, including enrichment of uranium on Iranian soil, and all sanctions lifted.

“This is going to be a critical stage today, and a critical process once we start implementing it,” Zarif told the Monitor. “I believe leaders must show courage and leadership in order to take this process forward, and that is what we expect from President Obama and Secretary Kerry – and that is what they should expect from us.”

Zarif said time was short for all sides as Iran negotiates with the P5+1 group (the US, Russia, China, Britain, France, and Germany). But opponents in Congress are demanding more sanctions on Iran, Israeli Prime Minister Benjamin Netanyahu has slammed a potential deal as a “historic mistake,” and hardliners in Iran have vociferously opposed any accommodation with the West.

“A day is a tall ambition [to finalize a text]; we will all try,” Zarif said. “I am ready to work as long as necessary, even to stay on if necessary to finish this, before detractors can have a chance to start creating obstacles in the way of agreement and mutual understanding – because some already have started, calling it a ‘historic mistake.’”

Mr. Kerry was in the region working on Israel-Palestinian peace when the decision was made overnight to travel to Geneva “in an effort to help narrow the differences,” said US State Department spokeswoman Jen Psaki. Kerry was invited by European Union High Representative Catherine Ashton, who leads negotiations on behalf of the P5+1, to take part in a trilateral meeting between Kerry and Zarif.

Kerry “is committed to doing anything he can” to narrow those differences, said a senior US official.

As the day’s events began, Ms. Ashton’s spokesman, Michael Mann, said “intense” diplomacy was under way to close a deal. News of Kerry’s decision to come sparked the arrival also of the three European foreign ministers.

But as hopes grew for a nuclear deal with Iran, so did the rhetoric of its opponents. Kerry canceled a photo opportunity with Mr. Netanyahu “in an attempt to avoid a public confrontation” as he left Israel for Geneva this morning, Haaretz reported.

But Netanyahu went before the cameras alone to declare that the Jewish state “utterly rejects” the proposed agreement.

Iran got "the deal of the century and the international community got a bad deal,” said Netanyahu. “Israel is not obliged by this agreement and Israel will do everything it needs to do to defend itself and the security of its people.”

Criticism likewise emerged in Iran – where Supreme Leader Ayatollah Ali Khamenei gave unprecedented support to Iran’s negotiating team in recent days, praising them as “sons of the revolution” in a bid to quiet hard-line opposition.

Addressing Friday prayers in Tehran, Ayatollah Movahedi Kermani warned that any deal in Geneva would hurt Iran.

"It's harmful to underestimate the enemy because they do nothing other than playing tricks," says Mr. Kermani, according to a translation by The Guardian. "The US secretary of State has pledged Netanyahu that he will not do a bad deal with Iran. It means that they will not agree to an agreement which is harmful for them, which means they will not make a good deal with Iran."

And new chances of a deal are alarming Iran hawks in Congress, who are now moving to speed up fresh sanctions measures – a step that the White House has fought against on Capitol Hill in recent weeks, saying that more sanctions now could disrupt sensitive talks.

Zarif said the presence of sanctions experts on the US team “indicates they are willing to deal with this issue. At least they have the equipment, the know-how, and the expertise to deal with the issue.”

But any new US sanctions would spark a fierce reaction in Tehran and deepen mutual mistrust that has long plagued these talks, Zarif warned.

“If we see lack of good faith, if we see attempts by Congress to torpedo this process, I don’t think our hands are tied,” said Zarif.

“My message to American lawmakers who have already started trying to prevent success of this is to look at their own record,” he said, adding that “it would have been a totally different story” today if the US had years ago accepted Iranian proposals to cap its nuclear program, when Iran had less than 200 centrifuges enriching uranium.

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“Now in 10 years of sanctions and intimidation, what the United States gained, what Congress gained, was 19,000 centrifuges spinning in Tehran. So that’s their record, and it’s a dismal record,” said Zarif.

“I think they’ve got to be careful not to repeat that. This is an opportunity that one year down the road, if not seized, will be wished for – and cannot be regained again.”