Iran and world powers are to return on Tuesday from a stormy session for what could be the last day of negotiations aimed at putting a peaceful halt to Tehran's disputed nuclear drive.
Western officials said on Monday that their patience was running out with Iran as its enrichment capabilities developed and the danger of it starting to produce material that could be made into nuclear weapons grew.
"If there are signs of progress that they want to move things forward, then we would do that," an EU official said in reference to a possible fourth round of negotiations at a future date.
But "we have to wait and see whether they come back with a positive attitude toward our proposals," the EU official added. "It is not in our interests to stall."
The West is waiting for Iran to respond to an offer made last month and repeated on Monday that would see it halt enrichment to the dangerous level of 20 per cent and ship out such existing material in return for some forms of aid.
Iran has countered with a demand that world powers recognise its "right to enrich" - something contradicting current UN resolutions - and rescind an oil embargo that the European Union intends to implement fully on July 1.
Tehran has most importantly rejected the idea of scaling back what it claims is a peaceful programme before it wins any concession - a point on which the world powers remain firm.
Western officials indicated that they were waiting for Iran to either tone down or somehow rephrase those demands because they remained deal breakers.
"The July 1 [oil embargo] deadline is law," the EU official said.
Negotiators from both host nation Russia - an Iranian partner that opposes sanctions - and the Islamic Republic suggested that the biggest problem lay in finding a way to deal with what now are mutually exclusive demands.
"How to bring these positions together is possibly the main difficulty," said Russia's Deputy Foreign Minister Sergei Ryabkov.
A member of the Iranian delegation also gave a downbeat assessment after his side made a power point presentation to teams from the five permanent members of the Security Council and Germany at a conference room in a Moscow hotel.
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"So far the atmosphere is not positive," the Iranian negotiator said. "Setting up the framework is the main problem."
The United States said the Moscow talks contained a "clear and direct" exchange of views on Iran's need to live up to its obligations.
"What took place today [is that] we made a number of clear points to the Iranians our concerns with their nuclear programme," said Ben Rhodes, a US deputy national security advisor, speaking at the G20 summit in Mexico.
"The Iranians came back today with a set of points in response to those concerns. The P5+1 one was unified in responding very forcefully that the onus is on the Iranians to come in line with their obligations."
'Restoring international confidence'
After meeting in Mexico, US President Barack Obama and Russian President Vladimir Putin jointly warned Iran it must comply with its nuclear obligations and "undertake serious efforts aimed at restoring international confidence in the exclusively peaceful nature of its nuclear programme".
The Iranians withdrew for private talks with senior Russian officials after the first full session to plot their next move.
The decision highlights the sway Russia still has over its Soviet-era ally and the role it can play in forcing a compromise out of its leadership that could stave off the threat of possible military attack.
Israel has warned repeatedly that it would strike its arch-foe the moment it felt Iran was approaching the point of no return on nuclear weapons.
Such an air raid would not only prompt a probable Iranian counter-strike and cause broader regional warfare but also almost inevitably draw the United States into a conflict it would at this stage prefer to avoid.