"The onus is on the Iranians" to convince the world they aren't seeking nuclear weapons, Vietor said. "Otherwise they will continue to face crippling sanctions and increased pressure."
The statement was released shortly after The New York Times reported that the U.S. and Iran have agreed in principle for the first time to negotiations. The paper said Iran has insisted the talks wait until after the Nov. 6 election.
On Sunday, Iranian Foreign Minister Ali Akbar Salehi refused to discuss the report, saying "it is not our responsibility to comment on what they say." However, he reiterated Iran's traditional stance that the country will hold no direct nuclear talks with the U.S. and all dialogue must be through the seven-nation framework.
He predicted the next round could be as soon as late November. Some analysts believe Iran is unlikely to make any additional overtures until after the U.S. election.
What Iran has put forward so far is effectively a two-track path.
One is seeking to secure an international pledge that it can continue uranium enrichment at some level, perhaps at lower grades for its lone energy-producing reactor. Iran is a member of the U.N. treaty overseeing nuclear technology and insists its uranium labs are within the guidelines. The U.N. nuclear watchdog agency, however, says Iran still needs to open up more sites for inspection.
The second goal is getting the West to roll back sanctions, which have cut into Iran's critical oil exports and cut off access to most international banking networks. A panic on exchange markets this month plunged Iran's rial nearly 40 percent to record lows against the U.S. dollar. The currency on Sunday again flirted with the all-time low of 35,500 rials to the dollar.
But the U.S. and allies would likely require major concessions from Iran — that the leadership may be unwilling to give — for any significant sanctions relief just as the economic squeeze appears to be bringing more pain. Last week, the European Union banned imports of Iranian natural gas and imposed other restrictions on trade and financial dealings — on top of a previous embargo of Iranian oil.
Iran's Foreign Ministry spokesman Ramin Mehmanparast described the new EU measures as "inhuman," but said they would not force any retreat on the country's nuclear program.
Sergey Barseqian, an independent Iranian political analyst, said the prospects for renewed talks are stuck in a "negative loop" with the West pressing harder on sanctions and Iran not willing to be portrayed as caving to the pressures.
"In practice, there is no initiative in Iran's hand" to restart talks, he said.
Iran's leaders also must juggle twin strategies.
For crowds at home, the Iranian hierarchy frames the country's nuclear advances as part of the country's modern identity. To a global audience, they display willingness for some degree of deal-making.
Last week, Supreme Leader Ayatollah Ali Khamenei said Iran will withstand the "conspiracies and tricks" of its foes, including sanctions and "talk of military aggression." But he also said Iran has never stepped away from the nuclear talks, which were last held in Moscow in June.
"The West ... says pressures on the Islamic Republic are aimed at forcing it to return to table of the talks," said Khamenei. "Have we ever left the negotiating table?"
The foreign ministry spokesman, Mehmanparast, last week reinforced the message, saying Iran was prepared to "show flexibility" over the 20 percent-enriched uranium if the West would give some ground.
"The other party needs to take measures to fully recognize Iran's nuclear rights and Iran's enrichment for peaceful purposes," he added.
The 20 percent fuel is needed for Tehran's medical research reactor, whose output includes isotopes for cancer treatment. A lower level of enrichment, 3.5 percent, is needed for Iran's electricity-producing reactor.
In the previous talks, the West demanded that Iran halt its 20 percent enrichment, ship out its stockpiles and shut down the enrichment labs at a subterranean facility south of Tehran that is believed to be virtually impervious to aerial attacks.
On Saturday, Iranian nuclear chief Fereidoun Abbasi told the official IRNA news agency that negotiations will "will be continued."
"There is no particular problem in the direction of the talks," he said.
Associated Press writer Nasser Karimi in Tehran, Iran, contributed to this report.
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