(CNN) -- Optimism is a term seldom used when describing the prospect of a nuclear deal with Iran.
Indeed nobody expects dramatic breakthroughs when Iran sits down on Tuesday in Geneva with six world powers for talks aimed at curbing its nuclear ambitions.
But a new feel-good tone from Tehran toward the United States and its western allies since the election of President Hassan Rouhani has diplomats hoping that an agreement might be possible after a decade-long standoff.
"The stakes are higher because the expectations are so much higher," one senior U.S. official said. "We aren't naive about the challenges but there is a new openness that is encouraging."
During his visit to the United Nations General Assembly in September, Rouhani's diplomatic approach raised hopes in the West of a thaw with Iran and progress in negotiations on its nuclear program.
Western powers believe Iran is trying to develop a nuclear weapon, but Tehran has long insisted that its intentions are peaceful.
Rouhani's visit culminated in a phone call with President Barack Obama and a meeting between Secretary of State John Kerry and Iranian Foreign Minister Javad Zarif.
It was the first such high-level contact between the two sworn enemies since Iran's 1979 revolution, which sent relations between the two into a deep freeze.
'Mistrust' colors chances for breakthrough
Despite the improved climate, a senior Obama administration official told reporters in Geneva that Washington is "not expecting a breakthrough overnight," given the history of mistrust between Iran and the West and the myriad of technical issues involved.
But the official hoped both sides can agree on a set of measures while they negotiate a comprehensive agreement.
The official added Iran previously used the time for negotiations to continue developing its nuclear program, but added "we cannot continue for that to be the case."
"There needs to be a confidence-building step that not only starts to get over the deep mistrust ... but also constrains Iran's program today and perhaps even takes it back a notch so that there is time on the clock to achieve that comprehensive agreement," the official said.
Rouhani, who replaced conservative hard-liner Mahmoud Ahmadinejad, was elected with a mandate to end the standoff and lift international sanctions that have nearly crippled its oil exports and access the international financial system.
The senior official briefing reporters said Washington wants to keep up that economic pressure as negotiations get under way.
"We believe that has got us to where we are today," the official said.
But the official said the Obama administration would be willing to consider quick relief on sanctions "targeted in proportion" to what Iran puts on the table should it be prepared to curtail the pace and scope of its uranium enrichment program, offer steps to improve transparency of its nuclear program, and address concerns over its stockpile of enriched uranium.
The official noted that sanctions experts from the State and Treasury departments are part of the American delegation in Geneva for the talks.
"If Iran is ready to go, we are ready to go," the official said.
'No one is naive'
Iran has signalled in recent days that it is ready to deal, with Zarif and his deputy, both saying the delegation is bringing a new proposal to the table.
As he headed to Geneva, Zarif repeated Iran's desire to reach an agreement.
"Tomorrow is the start of a difficult and relatively time-consuming way forward," the U.S.-educated veteran diplomat wrote in a post on his Facebook page late Sunday. "I am hopeful that by Wednesday we can reach agreement on a road map to find a path towards resolution."
The bloc of countries leading the diplomatic effort include five permanent U.N. Security Council members -- the United States, Russia, China, France and Britain -- plus Germany.
When Zarif met with the group last month, he presented what one senior U.S. official called "the most forward-leaning ideas" the Iranians have ever offered to dispel international concerns over its nuclear program since diplomacy began over a decade ago.
The senior U.S. official in Geneva said Zarif's "thoughtful presentation" in New York leads Washington to expect "something substantial" from the Iranian delegation at this week's talks.
But the official said the United States is "clear-eyed about what is very, very difficult work" and what is likely to prove a bumpy road full of hurdles.
"No one is naive," the official said.
In February, the group known as the P5 plus one offered Iran a package of economic incentives to abandon its nuclear ambitions.
In exchange for easing sanctions barring trade with Iran in gold and other precious metals and petrochemicals, the group wants Tehran to shut its underground enrichment facility at Fordo, near the holy city of Qom.
The group also wants Iran to ship its stockpile of uranium enriched to 20% purity, seen as a jumping part to producing weapons-grade uranium.
Iran wants the international community to acknowledge its right to enrich uranium under the Non-Proliferation Treaty.
The six negotiating countries group has balked at that, although privately some diplomats say a final deal could allow Iran to enrich uranium to a low purity, such as 3% - 5%.
They also proposed fuel for a medical reactor and easing sanctions on aviation spare parts as part of the deal.
The international community has expressed suspicions over enrichment, fearing Iran may secretly be transforming nuclear fuel into atomic bomb-grade materials.
Iran has never formally responded to the deal. It remains to be seen whether the group would ultimately be willing to sweeten the offer in the new climate, although U.S. officials said no new offer will be made to Iran at this week's talks.
Robert Einhorn, one of the U.S. negotiators with Iran until he left the State Department in May, said Iran's new negotiating team is motivated to negotiate more seriously than the previous team.
"This group really wants to reach a deal," said Einhorn, now at the Brookings Institution. "I don't know if our minimum requirements will overlap with their minimum requirements, but they are realistic enough to know sanctions won't be eased and eventually lifted without some curbs on its nuclear program.
Rouhani and Zarif appear to have the backing of Iran's Supreme Leader, Ayatollah Ali Khamenei, who has said he supports "heroic flexibility" in negotiations to ease sanctions.
But their room to maneuver may be limited.
Einhorn said the Iranians will need to show results fairly quickly to keep Khamenei's backing.
"The Supreme Leader has basically given them some running room," he said. "He's skeptical it will achieve anything, but has said they can go ahead and try this moderate approach and see where it gets them. But their mandate is not infinite. I don't know if there is a time limit but they genuinely need to show this moderate approach can work."
Labott reported from Washington and Carey from Geneva.