Racing against the clock, nuclear talks between Iran and six world powers appeared tough going Thursday with both sides warning of major differences as they tried to draft an accord.
The hoped-for agreement would see Iran scale back its nuclear programme, in order to ease fears it is seeking atomic weapons, and avert a conflict in the Middle East.
Iran in return wants painful UN and Western sanctions lifted. It denies wanting the bomb.
On the fourth day of a fifth round of talks in Vienna, Iran and the five permanent members of the UN Security Council plus Germany haggled over the wording of a deal, officials said.
But beyond agreeing a title for the accord, Iranian Foreign Minister Mohammad Javad Zarif said that "fundamental differences" were dividing the two sides and that the talks were "very difficult".
Top Iranian negotiator Abbas Araqchi said that they were "drafting the text of a final agreement, not the big issues but the general framework and the introduction."
A diplomat from one of the six world powers said that "some progress" was being made but that there remained "important differences of substance".
A second diplomat from a Western country said Iran was refusing to budge on most issues and echoed that drafting language on the "complex issues" had not begun.
"It is worrying that there is no evolution on the part of the Iranians on most subjects," the diplomat told AFP on condition of anonymity, with differences on key issue uranium enrichment "major".
Enrichment is front and centre of Western concerns about Iran's nuclear ambitions, as the process can produce both fuel for nuclear power plants and, when highly purified, the core of an atomic bomb.
The West wants Iran to slash the number of centrifuges, machines that enrich uranium, from 20,000, but Tehran wants to install many more in order, it says, to fuel future nuclear plants.
Other thorny issues include the duration of the mooted accord, the pace of any sanctions relief and a reactor being built at Arak that might give Iran weapons-grade plutonium.
Mark Fitzpatrick, a former US State Department official now at the International Institute for Strategic Studies in London, said putting off discussion of key issues was "hardly promising".
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"But at this stage it is not surprising. If there is going to be a breakthrough on the key issues, it won't come until the last moment," Fitzpatrick told AFP.
It was unclear late Thursday whether the talks would continue into a fifth day or even into Saturday.
- No catastrophe -
Araqchi told IRNA on Wednesday that choosing to push back a July 20 deadline for an accord -- when an interim deal struck in November expires -- "won't be a catastrophe".
But US President Barack Obama is not seen as keen, seeking ahead of November midterm US elections to silence accusations that the talks are merely giving Iran time to inch ever closer to the bomb.
"We are absolutely focused on July 20… We are not interested in talking about a rollover," the world power envoy said.
Complicating the process is the shared interest of Washington and Shiite Iran in seeing a lightning onslaught by Sunni rebels in Iraq stopped in its tracks.
On Monday US and Iranian officials briefly discussed the crisis on the sidelines in Vienna, although Washington said this would not be repeated.
On Wednesday a senior aide to Rouhani, his chief of staff Mohammad Nahavandian, appeared to say that success in the nuclear talks could pave the way for US-Iranian cooperation in Iraq.
"If that comes to a final resolution, then there might be opportunities for other issues to be discussed," Nahavandian said in Norway.
But US State Department spokeswoman Jen Psaki said Wednesday that any discussion of Iraq would be "entirely separate" from the nuclear talks.
"Any effort to connect the two is a nonstarter for the United States," Psaki told reporters.