WASHINGTON -- Ten years since the Islamic Republic began its controversial nuclear program, an historic diplomatic effort begins today in Vienna with the explicit aim of some parties to categorically end it, of others to contain it, and of one— Iran— to stall it, both for time and for relief from crippling sanctions on its shrinking economy.

Entering talks with such gaps between their end-state goals, officials from the United States, United Kingdom, France, Russia, China and Germany— the P5+1 parties involved in the talks— agree on at least one point: this negotiation will be one of the most difficult undertaken in decades.

"These next days this week are the beginning of what will be a complicated, difficult, and lengthy process," a senior US administration official told journalists in Vienna on Tuesday, before the resumption of talks.

"When the stakes are this high and the devil is truly in the details, one has to take the time required to ensure the confidence of the international community in the result."

On Monday, Iran's Supreme Leader Ayatollah Ali Khamenei lowered expectations before the talks in front a large crowd in Persian Azerbaijan, northwest of the capital.

"I am not optimistic about the talks, and it will not lead anywhere," Khamenei said. "But I am not opposed, either."

Putting the chances of success at just 50 percent, US President Barack Obama has said Iran will never unlearn how to build a nuclear weapon. But the international community can physically prevent its government from doing so, he asserts. The large-scale dismantling of Iran's program— the dismantling of centrifuges that enrich uranium, the dilution of uranium already enriched, the dismantling of Iran's plutonium reactor in Arak and curbs on its state-run research and development into nuclear weaponry, all with a decade or more of strict international oversight— will be key negotiating points pressed by the United States and its allies, France and the UK.

"We don’t know if, at the end of these six months, we will be able to achieve a comprehensive agreement, though we aim to," the official said. "It’s probably as likely that we won’t get an agreement as it is that we will."

"Probably with all of you," the official added, "we don’t have to worry about high expectations."

In an effort to maintain unity among the P5+1 with Moscow and Beijing, the White House has smothered a bill in the US Senate that would have triggered new sanctions tools against Iran should talks fail in six to twelve months time.

The Obama administration has also kept the prospect of a civilian nuclear-powered Iran on the table as a tolerable end-state, maintaining a carrot in the diplomatic process in an effort to demonstrate American interest in resolving the longstanding dispute peacefully.

Iran enters the talks vowing never to dismantle or shut down its expansive nuclear facilities, now spanning over 20,000 centrifuges wide.

Wendy Sherman, under Secretary of State for Political Affairs, leads the US delegation in Vienna once again after conducting bilateral talks with Iran through the autumn of 2013. Those talks led to an interim deal between world powers and Iran freezing much of its nuclear program, in exchange for $7 billion in sanctions relief from the global community, which began in January and will continue for six months while the parties attempt to negotiate a final settlement.

Under the interim deal, formally known as the Joint Plan of Action, Iran may continue research into advanced centrifuge prototypes that would allow them to enrich uranium exponentially more efficiently than their standard model.

A monthly update by the United Nations atomic watchdog is expected to show later this week that Iran is thus far complying with the JPOA. But the International Atomic Energy Agency is also likely to report that Iran's stockpile of lower-grade refined uranium has increased in recent months, one diplomat involved in the negotiations said on Monday.

A team of aides from the National Security Council, State Department and Treasury Department have accompanied Sherman, guiding her on security, diplomatic and sanctions matters.

Brooke Anderson joins the team as a special envoy on the Iran nuclear negotiations, leaving her post as chief of staff to National Security Advisor Susan Rice. Anderson previously worked toward nuclear nonproliferation at the UN.

Reuters contributed to this report.

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