Iran became more adamant Monday that the world must ease the sanctions choking off its oil sales before it will curb activities that could be used to make nuclear arms, diplomats said. But with six world powers insisting that Tehran take the first conciliatory step, fears grew that talks in Moscow would fail.

The diplomats said the Islamic Republic had asked the six world powers it is meeting to talk about "comprehensive sanctions relief," along with any consideration of their request that Tehran stop enriching uranium to a level just steps away from the purity needed to arm nuclear missiles.

But the six world powers — the United States, Britain, France, Russia, China and Germany — insist that any lessening of sanctions can come only after Tehran curbs that kind of enrichment, and the first of two days of talks ended Monday on a downbeat note.

The talks are being hosted by EU top foreign policy official Catherine Ashton and her spokesman, Michael Mann, spoke of "pretty tough" going, with Iran presenting arguments and objections that went over ground already covered in two previous inconclusive meetings in April and May.

The six nations had asked the Iranians to respond concretely to their demand to curb higher-level enrichment. While Mann said that topic was "addressed," he refused to qualify whether the Iranian response met the expectations of the world powers.

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    The United States and others suspect that Iran is enriching uranium to develop nuclear weapons. Tehran denies this, saying all of its nuclear goals are peaceful and it is enriching solely to make reactor fuel and medical isotopes. But fears have been fueled by Tehran's refusal to stop enriching or accept uranium from abroad.

    While the Islamic Republic has previously mentioned lifting sanctions or staying pending ones, one of the diplomats said Monday's request was the most direct to date. That appeared to reflect the mounting pain caused by accumulating sanctions, particularly international embargoes on Iran's oil sales.

    In addition to longer-term U.N. and other sanctions, Tehran is now being squeezed by the widening international embargo on its oil sales, which make up more than 90 percent of its foreign currency earnings.

    Sanctions levied by the U.S. have already cut exports of Iranian crude from about 2.5 million barrels a day last year to between 1.2 and 1.8 million barrels now, according to estimates by U.S. officials. A European Union embargo on Iranian crude that starts July 1 will tighten the squeeze.

    Comments by Ali Bagheri, the No. 2 on the Iranian delegation, reflected the gap between the two sides' priorities.

    "We elaborated in detail ... the illegality of referring Iran's nuclear issue to the U.N. Security Council and issuance of U.N. Security Council resolutions," he told reporters, referring to Security Council demands — enforced by sanctions — that Iran stop enriching.

    Diplomats from several nations meeting with Iran in Moscow depict the talks as significant. They say it could be the last in a series and if negotiators fail to make headway in persuading Tehran to stop higher-grade uranium enrichment, it's unclear if or when new talks would occur.

    While Iran wants the other side to recognize its right to enrich and blink first by easing sanctions, the six nations say the onus is on Tehran to show it is ready to compromise.

    Because it lives off its oil sales, Iran would be most immediately hurt by a lack of progress in Moscow followed by any long hiatus in new negotiations. But the White House also stands to lose.

    Failed talks at Moscow with no immediate prospect of new meetings would expose President Barack Obama to criticism of weakness in dealing with Iran from Mitt Romney, his U.S. Republican presidential challenger, and from Israel, which has threatened to attack the Islamic Republic's nuclear installations should diplomacy fail.

    It is unclear if the Jewish state would actually make good on such a threat. But any military move would likely draw in the U.S. and widen the conflict through much of the Mideast, spiking the price of oil upward and further hurting the already ailing world economy.

    The six nations formally are only prepared to ease restrictions on airplane parts for Iran's outmoded, mostly U.S.-produced civilian fleet and are offering technical help with aspects of Iran's nuclear program that cannot be used for military purposes.

    While not budging on lifting existing sanctions or those already decided upon, diplomats familiar with the talks told the AP, however, the six are also prepared to guarantee that no new U.N. penalties will be enacted if Tehran compromises enough. The diplomats demanded anonymity because that possible offer has not yet been formally made.

    For Iran, the main formal demand remains international recognition of its right to enrich and related issues — with increasing emphasis on sanctions relief.

    The six are pressing the Islamic Republic to stop higher enrichment to 20 percent purity because at that level the material can be turned into weapons grade uranium much more quickly.

    They also want Fordo, the underground Iranian facility where most of this enrichment is taking place, shut down and for Iran to ship out its higher-grade stockpile. Fordo is of special concern because it might be impervious to air attacks, a possible last-resort response to any Iranian bomb in the making.

    An Iranian delegate, who demanded anonymity because he was not authorized to discuss Iran's position, told The Associated Press that his country was ready to compromise but only if the six accepted Tehran's right to enrich. In turn, he said Iran may agree to consider suspending 20 percent level enrichment as a voluntary, temporary measure.

    "Our minimum demand ... is for them to recognize our right to uranium enrichment," he said. "If this is not accepted by the other side, then the talks will definitely collapse."


    Associated Press writer Ali Akbar Dareini contributed.