World powers resume crisis talks with Iran on Monday amid hope that a crippling oil embargo and pressure from host Russia will finally force the Islamic Republic to scale back its nuclear drive.
The two-day meeting follows a bruising May session in Baghdad during which Iran nearly walked out of negotiations aimed ultimately at keeping it from joining the exclusive club of nations with an atomic bomb.
Host Russia however is keen to flex its diplomatic muscle and make Iran an example of how Moscow's influence over Soviet-era partners could be used to avoid foreign military intervention in the 16-month crisis in Syria.
"There are reasons to believe that the next step will be taken in Moscow," Russia's Deputy Foreign Sergei Ryabkov said.
Failure in Moscow could leave the process in tatters and raise the threat of air raids from arch-foe Israel -- a fateful scenario in which broader conflict would lead to a spike in oil prices that could tip over the world's teetering economy.
But a July 1 deadline for a full EU oil embargo and the June 28 rollout of tough US sanctions against a host of Iranian oil clients is providing added pressure for Tehran to bargain more seriously.
Two of the biggest bones of contention involve the speed with which world powers lift existing sanctions and the recognition of Iran's "right to enrich" uranium.
The latter is emerging as a key demand that Iranian negotiator Saeed Jalili is likely to present to EU foreign policy chief Catherine Ashton when she represents the five permanent UN Security Council members and Germany in Moscow.
"We expect that Iran's right to nuclear technologies, including uranium enrichment, will be recognised and respected," Jalili told Russia's RT state-run world news channel in comments translated from Farsi.
Iran for its part "has the capacities to cooperate in disarmament and nuclear non-proliferation, so these capacities should be used by the international community," Jalili said in Friday's broadcast.
"I think that addressing these two issues will help to advance the negotiations."
In Brussels, Ashton spoke by telephone with Russian Foreign Minister Lavrov.
She said in a statement issued on Sunday evening that the pair "agreed that it was vital to move the talks forward to reassure the international community of the exclusively peaceful nature of Iran's nuclear programme".
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Diplomats said Iran had agreed to discuss the idea of limits to its enrichment programme under a proposal initially outlined in Baghdad.
"Their message on enrichment has been received," said a Western diplomat close to the negotiations.
"I think that much will depend on how Iran reacts to our proposals as well. But we're ready to discuss theirs," the Western diplomat said.
But Western officials have also made clear that Iran's current position would leave them no choice but to go ahead with the oil sanctions while considering new ones in the months to come.
They add that the two sides still remain far apart despite the mounting pressure.
The offer outlined by the powers last month and under discussion in Moscow would see Iran stop enriching uranium to 20 percent -- seen as being just steps away from weapons-grade -- and ship out its existing stock while shuttering its Fordo bunker.
The nuclear enrichment site is buried deep in the Iranian mountains and is believed to be bunker-buster proof.
The tough terms would not lead to the quick lifting of sanctions but instead see the West extend some forms of peaceful nuclear energy cooperation and provide assistance for Iran's battered aircraft industry.
Europe would also help Iran export oil to key client Asia by easing an EU ban on tanker insurance.
Iran has previously scoffed at the idea of accepting only reactor fuel and civil aviation parts in immediate return.
But calls are growing on US President Barack Obama from both Israel and the US Congress ahead of his November re-election bid to reject any compromise.
A bipartisan letter signed by 44 senators urged Obama on Friday to cut off negotiations unless Iran agrees to shutter the Fordo bunker and halt its enrichment programme outright.
"The biggest hurdle to a nuclear deal right now is the absence of political will in Washington," said former Iranian delegation adviser Kaveh Afrasiabi.
But "I am cautiously optimistic and expect even a mini breakthrough provided that the Western governments display the needed flexibility," Afrasiabi told AFP.