Analysts Sunday praised a ground-breaking interim deal with Iran to rein in its nuclear program in return for sanctions relief, but warned negotiations for a full agreement will prove even tougher.
"The interim deal is a strong one, which achieves a broader array of constraints and verification on Iran's nuclear program than ever previously contemplated," said Suzanne Maloney, senior fellow with the Brookings Institution.
She stressed that the six-month accord tied Tehran "to an ongoing diplomatic process whose primary rewards remain deferred until a far more ambitious agreement can be achieved."
After almost a decade of stalemate in negotiations between Iran and the so-called P5+1 group -- Britain, China, France, Germany, Russia and the United States -- things have moved at breakneck speed since Iran's President Hassan Rouhani took office in August and was hailed by some as a moderate.
Three rounds of negotiations since September led to a pre-dawn deal in Geneva.
"I think this has been a hard fought, effective and meaningful negotiation and has provided the kind of assurances that the negotiators need to have a legitimate, comprehensive discussion with Iran about the entirety of its nuclear program," said Joel Rubin, director of policy for the non-governmental Ploughshares Fund.
The deal had put "time back on the clock" as the world tries to stop Iran getting to a so-called breakout capability where it could deploy a nuclear weapon, Rubin said, adding that Tehran's acceptance of daily inspections of by UN inspectors of its Natanz and Fordo plants was "unprecedented" as was an accord to stop work on its heavy water reactor at Arak.
"It's quite remarkable. If you had told me a few months ago that Iran would agree through diplomacy to eliminate its 20 percent stockpile, to stop construction and any progress of Arak, to allow for intrusive and daily monitoring, I would tell you that that is an achievement for our security," Rubin told AFP.
But there was also a note of caution that the next round of negotiations towards a fully comprehensive deal could prove even tougher.
Kenneth Pollack, senior fellow for the Saban Center for Middle East Policy at Brookings, said it was "plausible" to think that Iran and the international community could now work out a full deal over the next six months, but "we should recognize that that step may prove far, far more difficult than the agreement just negotiated.
"The complexities are far greater. The concessions that both sides will be required to make will be far more painful," he cautioned.
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One potential stumbling block remains Iran's insistence on its right to enrich uranium, which can be used to arm a nuclear warhead. Iranian Foreign Minister Mohammed Javad Zarif told reporters in Geneva that the accord had a "clear reference that enrichment will continue".
"If Tehran insists on standing on principle--especially on its 'right' to enrich and the lifting of 'all' sanctions--such a resolution may well prove impossible," Pollack said.
Other analysts pointed to the fact that the six-month deal did not deal with any actual dismantling of Iran's nuclear program, including its estimated 19,000 centrifuges used for enrichment.
Hailing the deal as a breakthrough, French expert Bruno Tertrais from the Foundation for Strategic Research, warned however it remained a "limited" agreement.
"This deal says nothing about its militarization activities, its studies and research and the experiments Iran may have carried out for developing a nuclear device," Tertrais said.
"In some ways that is the most important thing... because the heart of the problem is not the enrichment, it's what you do with it."
Anger from Israel, an arch-foe of Iran, as well as from skeptical US lawmakers eager to press ahead with more sanctions on the Islamic republic, may also signal trouble for the negotiators going forward.
The National Security Network said in a statement that it "believes that now is not the time for more sanctions and that a deal where Iran makes the choice not to pursue a weapon remains the best way to achieve our goal of a non-nuclear Islamic Republic of Iran."
But Reuel Marc Gerecht warned the interim deal most likely contained all that Iran was willing to concede.
"I think what has to be remembered is right now we're at the high water mark of our leverage. I think in six months time we're going to discover we don't have much at all," he told CNN.
A combination of circumstances had come together to create give "a window" for a deal, said Rubin, pointing to Rouhani's surprise election, and the crippling sanctions on Iran which have left it running out of cash.
"Kerry said the hard work comes now and in a way it really does. This is going to challenge all of the feelings, and conceptions and ideologies and emotions that have been pent up in the US, in the West and in Israel and elsewhere for decades. It's going to be a very, very hard task," he said.