(Reuters) - Iran told six big powers on Friday it would not accept their "excessive demands" after the latest talks on lifting sanctions against Tehran in exchange for curbs on its nuclear work yielded no breakthrough, with a deadline for a deal just a month away.

U.S. Under Secretary of State for Political Affairs Wendy Sherman said it was Iran that would need to shift its position: "What is still unclear is if Iran is really ready and willing to take all the necessary steps to assure the world that its nuclear programme is and will remain exclusively peaceful."

The stakes are high in the Vienna talks, which will resume on July 2, as the powers seek a negotiated solution to a more-than-decade-long standoff with Iran that has raised fears of a new Middle East war and a regional nuclear arms race.

Sherman noted at the end of five days of negotiations in the Austrian capital that Tehran had always maintained that it wants only civilian nuclear energy. "If that is indeed the case, then a good agreement is obtainable," the U.S. delegation chief said.

Iran and the United States, Russia, China, France, Britain and Germany are striving for a comprehensive settlement by July 20, a deadline set as part of an interim deal struck last year.

A six-month extension of the talks is a possibility but could be politically difficult for the United States, since the administration of President Barack Obama would almost certainly seek the approval of Congress, where hawkish lawmakers are suspicious of Iran and dislike the idea of engagement with it.

Diplomats from the six powers told Reuters earlier in the week that the most formidable dispute in the talks was over the number of centrifuges Tehran will be allowed to keep to enrich uranium under any deal.

Western officials say that the six powers want this number to be in the low thousands, below the capacity that could allow Iran to quickly accumulate enough material for a nuclear bomb.

Iran insists on tens of thousands of centrifuges to churn out fuel for a future network of civilian nuclear power stations, although this would take many years to build.

Iranian Foreign Minister Mohammad Javad Zarif highlighted the wide gulf between the sides, urging the six nations to "abandon excessive demands which will not be accepted by Iran".

"Still we have not overcome disputes about major issues," he told reporters as five days of negotiations in Vienna wound up. "There has been progress, but major disputes remain."

He made clear there was no agreement yet between Iran and the six on a draft text of an agreement. A senior Chinese official said the two sides had put together a "textual framework", though gave no details.

"The fact that (we came up) with this text is progress ... in procedural terms," China's Wang Qun told reporters.

Sherman described the text as a "working document" that is "heavily bracketed" due to remaining disagreements, making clear much work remains to reach an accord.

POWERS WANT 20-YEAR DEAL, IRAN WANTS 5 YEARS

So far, diplomats said, Russia and China - traditionally more accommodating of Iran's nuclear stance - have backed up the U.S. and European demands on Tehran's centrifuge capacity, though they support the idea of moving more swiftly to ease the sanctions that have crippled the oil-dependent Iranian economy.

A senior diplomat from one of the major powers said all six were united in their positions on the permissible scope of Iran's enrichment programme and that they had presented "pretty detailed" proposals on that issue.

"There are very, very difficult decisions to be taken here by Iran," said a senior U.S. official, asking for anonymity.

There are other sticking points in addition to centrifuges. One official from the six told Reuters that the Western powers want the duration of any agreement to be two decades, while Tehran has said it would be willing to accept five years.

Still, senior officials close to the talks said both sides are keen for a deal. Perhaps signalling its desire for a successful outcome, Iran has acted to eliminate virtually all of its most sensitive stockpile of enriched uranium gas, the U.N. nuclear watchdog reported on Friday.

That requirement was included in the interim deal reached in Geneva last November that bought time for the current negotiations on a long-term agreement.

A spokesman for European Union foreign policy chief Catherine Ashton, who coordinates the talks, said only that the two sides had begun drafting the text of a deal during their fifth round of negotiations this year.

"We have worked extremely hard all week to develop elements we can bring together when we meet for the next round in Vienna, beginning on July 2," Michael Mann said in a statement.

Iran denies any nuclear arms ambitions and demands crippling economic sanctions, eased slightly in recent months, be removed fast under any settlement - something Western governments are loath to do too soon, believing Tehran will otherwise lose incentive to comply fully with terms of a final deal.

Other issues awaiting resolution include the breadth and depth of U.N. nuclear watchdog monitoring of Iranian nuclear sites and the future of Iran's planned Arak research reactor, a potential source of plutonium for atomic bombs. Iran says the reactor will make isotopes for medical care and agriculture.

Israel's government, which has vocally opposed diplomacy with its arch-enemy Iran, has suggested it could bomb Iranian atomic facilities if diplomacy fails to head off the risk of a nuclear-armed Iran. Tehran says it is Israel's presumed nuclear arsenal that is the main threat to regional peace and stability.

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