Direct meetings this week between Iran and world powers, including the US, aim to deliver what diplomats are calling a "trust mechanism" meant to ensure both sides honour a nuclear deal.
The announcement of separate talks with the United States, Russia, France and Germany -- all members of the P5+1 that is negotiating with Iran -- underscores that serious differences remain.
Current and former diplomats say the problem is a failure to overlap positions and interests, which must be reconciled by a July 20 deadline.
Iran has in the past few weeks repeatedly declared its "inalienable" right to pursue a nuclear programme for peaceful purposes, while insisting that sanctions be lifted.
By contrast, the United States and other members of the P5+1 have said almost nothing publicly, indicating only that there are significant gaps between the two negotiating teams.
While the Western powers and Iran both say they want an agreement, neither is yet willing to cede sufficient ground.
"The Iranians want a robust civil nuclear programme that would give them a rapid nuclear breakout capability and a future nuclear weapons option," a former US negotiator told AFP.
"But America wants to keep Iran as far from the nuclear weapons threshold as possible," he said, admitting to a deficit of understanding made worse by a climate of suspicion.
The solution is an agreement that neither side can later back out of, but despite the talk of a good atmosphere and constructive negotiations, a bitter divide remains at the heart of the negotiations.
- Talks enter intensive phase -
"The aim is to ensure that Iran does not get a bomb," said a Western diplomatic source based in Tehran, conceding that the talks have entered an "intensive" phase.
"They know and we know that there is a little time left. We have a lack of trust between the two sides and we have to find a mechanism to build trust. This is hard to achieve."
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Talks with US officials ended in Geneva on Tuesday night, with lran's top negotiator reiterating that "divergencies" remain. Meetings with France, in Geneva, and Russia, in Rome, are scheduled to take place on Wednesday.
On Wednesday in Tehran, foreign ministry spokeswoman Marzieh Afkham noted the talks, saying Iran has always asked "for realism and restoration of our nuclear rights," remarking that the Islamic republic is "against any weapons of mass destruction, or proliferation."
The bilateral meetings with four of the P5+1 powers -- no meetings with Britain or China are yet planned -- aim to bridge the gap before the next round of main talks in Vienna, between June 16-20.
Gholam-Ali Khoshroo, a former deputy foreign minister and member of Iran's nuclear negotiating team between 2003-2005, agreed on the need for trust-building measures and said direct talks could iron out differences with specific P5+1 nations.
"The nearer we get to a comprehensive agreement the more we need to examine all the issues," he said, identifying the repeal of various sanctions and limits on Iran's enrichment capability as pertinent.
"During a meeting between seven countries (the P5+1 plus Iran) it is not possible to discuss precise details of all the questions."
- Concern at US midterm elections -
Another Iranian former negotiator, on condition of anonymity, was more specific, identifying US President Barack Obama's position ahead of midterm elections in November as a big concern for Tehran.
"The main problem is Obama has failed to demonstrate the necessary power and political reserves needed to solve the problem," said the ex-diplomat, noting rising pessimism about the US position.
"They know that if the problem is not resolved in July or in August or September, it is very possible with a change in Congress, that it could be harder to accept the commitments given to Tehran," he added.
The fear of a deal being signed and then going sour was likely raised in the Geneva meetings, a Western diplomat in Tehran said.
"If they have expressed worries that is a good thing," he said, noting that Iran has abided by the commitments it gave in the interim Geneva agreement which expires on July 20.
"It also shows that the (Iranian) taboo of speaking openly to the US has gone."