US Secretary of State John Kerry and foreign ministers of other major powers converged yesterday to lend their weight to the Iran nuclear talks after envoys reported progress on key issues blocking an interim agreement to curb the Iranian programme in return for limited sanctions relief.

Mr Kerry, who arrived yesterday, said before leaving Washington that he had no particular expectation that an agreement could be reached this week.

But after talking with top EU diplomat Catherine Ashton on Friday, he decided to travel to Geneva, Switzerland, to help negotiators narrow their differences.

He was joining foreign ministers from the five other countries – Russia, Britain, France, China and Germany – that have been negotiating with Iran over its nuclear programme for years.

China’s Xinhua news agency quoted Chinese Foreign Ministry Spokesman Hong Lei as saying that the talks had reached “the final moment”.

French Foreign Minister Laurent Fabius told reporters he wanted “a deal – but a solid deal – and I am here to work toward that end”. France’s concern that the negotiators were rushing into a flawed deal with Iran helped delay an agreement during a session nearly two weeks ago.

The ministers rushed to Geneva after diplomats said on Friday that Iran’s Foreign Minister Mohammed Javad Zarif and Catherine Ashton had made progress on a key sticking point – Iran’s claim to a right to produce nuclear fuel through uranium enrichment.

Enrichment is a controversial issue because it can be used both to make reactor fuel and to make nuclear weapons. Iran argues it is enriching only for power, and scientific and medical purposes, and says it has no interest in nuclear arms.

Washington and its allies point to Tehran’s earlier efforts to hide enrichment and allege it worked on developing such weapons.

Iran has insisted on that right throughout almost a decade of mostly fruitless negotiations. But Mr Zarif last weekend indicated that Iran is ready to sign a deal that does not expressly state that claim.

Iranian hard-liners are suspicious of talk of nuclear compromise since moderate President Hassan Rouhani took office in September, fearing his team will give not get enough in terms of sanctions relief.

Several US senators – both Democrat and Republican – have voiced displeasure with the parameters of the potential agreement, arguing that the US and its partners are offering too much for something short of a full freeze on uranium enrichment.

Mr Kerry said before leaving Washington that he had no particular expectation that an agreement could be reached this week

On Wednesday, Iran’s supreme leader, Ayatollah Ali Khamenei, said his country would never compromise on “red lines”. Since then Tehran has publicly reverted to its original stance – that the six powers must recognise uranium enrichment as Iran’s right, despite strong opposition by Israel and within the US Congress.

Comments from Iranian officials in Geneva indicated that reverting to tough talk on enrichment may be at least partially meant for home consumption.

In Geneva, a senior Iranian negotiator said the Iranian claim to the right to enrich did not need to be explicitly recognised in any initial deal, despite Mr Khamenei’s comment, adding that the supreme leader was not planning to intervene in the talks.

He did suggest, however, that language on that point remained difficult and that there were other differences.

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