World powers were set Tuesday to revive stalled talks with Iran over its controversial nuclear ambitions, with all eyes on what Tehran's top diplomat would put on the table.
Two days of closed-door negotiations were due to begin at 9:30 am (0730 GMT) at the United Nations' European base in the Swiss city of Geneva.
On the eve of the meeting, however, both sides downplayed chances of any major advance despite hopes raised by the more moderate government at the helm of the Islamic republic since conservative Mahmoud Ahmadinejad wrapped up two terms in office.
His successor President Hassan Rouhani took office in August, promising transparency on the nuclear drive and engagement with the world to eventually lift the sanctions strangling Iran's economy.
His foreign minister Mohammad Javad Zarif -- set to present Iran's positions to the P5+1 group of the United States, Britain, France, China and Russia plus Germany -- said Monday that its three-step proposal could be implemented "within a year."
While not going into details, he said the initial step could be achieved "within a month, or two, or even less."
European Union foreign policy chief Catherine Ashton, who chairs the talks, said Monday she had come "with cautious optimism but a real sense of determination," with the goal to go into details of proposals and explore possibilities.
Ahead of the meeting with Iran's team -- expected to be led for the bulk of the talks by deputy foreign minister Abbas Araqchi -- a senior US administration official also said detail would be the key.
"We are quite ready to move. But it depends what they put on the table," the official told reporters.
"We are hopeful, but that has to be tested with concrete, verifiable actions," the official said.
"In the past, Iran has taken the negotiated time and just kept moving forward with its nuclear programme. We cannot allow that to be the case."
Zarif for his part admitted to difficulties in the negotiations, on hold since a round in April in the Kazakh city of Almaty where Iran refused to curb some sensitive enrichment activities in exchange for a moderate relief of sanctions.
"The nuclear issue cannot be resolved in one session, as mistrust has been accumulated over years," he said.
"I am not pessimistic about the talks, but we need to see the good intentions and political will of the other side in action," he said.
Western powers and Israel suspect Iran is developing a military capability via what it calls a peaceful atomic energy programme, a claim vehemently denied by Tehran.
Iran has already drawn its red lines for the talks, saying it will not accept any demand to suspend uranium enrichment or ship out stockpiles of purified material.
The US official highlighted sources of concern in the West, including "basis of the enrichment programme, transparency of the programme and the stockpile of enriched uranium" as well as a heavy water reactor in Arak.
The Arak facility could provide a second path to the bomb by producing plutonium, which the US official called "a subject of enormous concern."
A first meeting between Zarif and his counterparts from the six powers took place last month on the sidelines of the UN General Assembly, accompanied by a landmark bilateral meeting with US Secretary of State John Kerry.
After meeting Ashton in London on Sunday, Kerry said the window for diplomacy with Iran was "cracking open."
Israel has repeatedly called on its US ally not to fall for "sweet talk" from the new Iranian president, insisting that actions not words are essential to ensure Tehran never acquires an atomic bomb.
Tackling such concerns, Kerry underlined Sunday that Washington meant what it said when it insisted it would never allow room for a nuclear-armed Iran.
"I believe firmly that no deal is better than a bad deal," he said.
Iran currently has 6,774 kilogrammes of low-enriched uranium, and a lesser quantity of enriched uranium.
The enriched uranium sparks the greatest concern in the West and Israel, which fear Tehran could divert some for further enrichment towards a level required for nuclear weapons.