World powers meet negotiators from Iran in Kazakhstan on Friday in the hope of seizing a possible window of opportunity presented by rare signs of progress in February talks over Tehran's disputed nuclear programme.
The two-day talks at the foot of the snow-capped Tien Shan mountains in Almaty come five weeks after the two sides met at the same venue for what Tehran described as "positive" and the world powers more cooly as "useful" discussions.
The "P5+1" world powers -- Germany plus United Nations Security Council permanent members Britain, China, France, Russia and the United States -- are seeking to coax Iran into softening years of defiance over its disputed nuclear programme, using a combination of demands and encouragement including a slackening of sanctions.
European Union foreign policy chief Catherine Ashton, the lead negotiator for the P5+1, said Wednesday that Iran needed to act on the world powers' revised offer, which involves easing some sanctions in return for concessions over uranium enrichment activities.
Ashton, whom Iran thanked for her more "understanding" stance last time round, said she was "cautiously optimistic" going into the new talks.
"But I'm also very clear that it's very important to get a response," she added.
"I really do hope that Iran will now... consider the proposal we put on the table and respond to it."
"The language surrounding the talks has improved on both sides," said Trita Parsi of the National Iranian American Council lobby group in Washington.
"But the negotiating process is still fragile and can easily be scuttled."
Iran has been open to the possibility of halting 20-percent uranium enrichment that puts it within scientific striking distance of getting the material needed for a nuclear bomb.
But it has refused the other demands of shipping out the bulk of its current stock of such material and halting production at the Fordo mountain bunker where this uranium is enriched.
The nuclear cooperation with international inspectors from the United Nations meanwhile has been sporadic at best. Tehran insists its nuclear drive is solely aimed at producing energy.
US President Barack Obama last month said Iran was just "over a year or so" away from making a nuclear weapon -- if the political decision to do so was ever reached.
The comment appears aimed at putting pressure on Israel, which has never ruled out pre-emptive air strikes against Iran, to hold back its firepower and give the negotiations more time.
But Israeli Prime Minister Benjamin Netanyahu voiced concern Wednesday that the talks were just allowing Iran more time to build a bomb.
"This model of a country talking, but at the same time developing nuclear weapons; threatening and at the same time developing nuclear weapons and threatening the use of nuclear weapons, we cannot allow this to happen in Iran," a statement from Netanyahu's office cited him as saying.
Few analysts expect an outright breakthrough at least until after Iran holds elections on June 14 to replace President Mahmoud Ahmadinejad, who after serving two terms is not allowed to stand again.
"The general thinking is that until the elections, there won't really be a basis for a consensus on the kind of concessions and compromises that would be needed to strike a deal," said Mark Fitzpatrick of the London-based International Institute for Strategic Studies.
"The one reason that there might be a breakthrough is if Ahmadinejad wants to be the one who achieves it," Fitzpatrick added.
Sanctions have been severely hurting the Iranian economy, with inflation at 31.5 percent and the currency losing more than two-thirds of its value since 2012.
One key event could be bilateral sit-down talks between US negotiator Wendy Sherman and her Iranian counterpart Saeed Jalili, which would mark an unprecedented encounter between the two foes in the decade-long nuclear standoff.
Iran's supreme leader Ayatollah Ali Khamenei -- the ultimate decision-maker on all aspects of society -- issued a rather vague comment on March 21 in which he said he was "not optimistic about these (direct) talks but not opposed to them either."
"Khamenei's comments might give Iranian negotiators the latitude to meet briefly with US diplomats," said Shashank Joshi at the Royal United Services Institute in London.
"Even a brief meeting would set a precedent."
Russian Deputy Foreign Minister Igor Morgulov told the Interfax news agency Wednesday that world powers hoped that the "package of demands and stimuli" on offer would be the "basis" for progress in the negotiations.
The P5+1 have been tight-lipped about what types of sanctions they are willing to lift.
But they are believed to concern Iran's metals trade and some very small financial transactions.