Iran and the United States, Russia, China, France, Britain and Germany are striving for a comprehensive settlement by July 20 that would defuse fears of a new Middle East war over a dispute that has stoked geo-strategic tensions for a decade.
A six-month extension has been mooted, but this would raise jitters since Israel has warned it could bomb Iranian nuclear sites if it deems diplomacy incapable of reining in Tehran.
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Iranian Foreign Minister Mohammad Javad Zarif highlighted the stubborn 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," Zarif 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.
Diplomats from the six powers told Reuters earlier in the week that one of the most difficult issues in the talks was 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 to prevent any Iranian dash to a nuclear bomb-making capability. Iran insists on tens of thousands of centrifuges to make fuel for what is says is a panned network of civilian nuclear power stations.
So far, diplomats said, Russia and China - traditionally more accommodating of Iran's nuclear stance - have backed up the US and European demands on Tehran's centrifuge program.
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 program and that they had presented "pretty detailed" proposals on that issue.
Still, senior officials close to the talks said both sides want a deal. Perhaps signalling its desire for a successful end of the negotiations, Iran has acted to eliminate virtually all of its most sensitive stockpile of enriched uranium gas, the UN nuclear watchdog reported on Friday.
That requirement was included in a landmark nuclear deal Iran struck with six world powers in Geneva last November to buy time for the current talks 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.
"We presented each other with a number of ideas on a range of issues, and we have begun the drafting process."
The powers are seeking a settlement that would limit Iran's nuclear program, subject it to stricter UN inspections and gradually lift sanctions impairing Iran's oil-based economy. Iran wants all sanctions removed swiftly after any accord.
A diplomat with one of the six powers said on Thursday that some progress had been made in this week's round but "we have not concluded a big element of the negotiation".
"There are still a lot of differences between the two sides and they are important differences of substance."
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 Iran will otherwise lose incentive to comply fully with terms of a final deal.
Other complex issues remain to be resolved as well, including the breadth and depth of UN nuclear watchdog monitoring of Iranian nuclear sites, how long any agreement should run and the future of Iran's planned Arak research reactor, a potential source of plutonium for atomic bombs.