Iran nuclear talks: Hague bids to close 'narrow gaps'
The UK's foreign secretary has said "narrow gaps" remain between Iran and the six world powers meeting in Geneva to discuss Tehran's nuclear programme.
Negotiators from the UK, US, Russia, China, France and Germany want Iran to stop enriching uranium in return for a loosening of sanctions.
Iran's deputy foreign minister Abbas Araqchi said Iran was insisting on a "right to enrichment" in any deal.
William Hague said any deal with Iran reached in Geneva must be thorough.
Tehran denies repeated claims by Western governments that it is seeking to develop nuclear weapons, and insists it must be allowed to enrich uranium for power stations.
Some US politicians say they will push for more sanctions if the talks fail.
Negotiators have been working since Wednesday to try to find an agreement that is acceptable to both sides.
The talks had been scheduled to finish on Friday but were extended. Foreign ministers from the so-called P5+1 group of nations joined on Saturday, amid hopes of a breakthrough.
US officials said Secretary of State John Kerry, who arrived in Geneva early on Saturday, had the goal of "continuing to help narrow the differences and move closer to an agreement".
Mr Kerry's participation in itself does not prove a deal is at hand, but it does show that the talks may have reached a critical stage, says the BBC's James Reynolds in Geneva.
Mr Hague said that a deal would be done only if it was a "truly worthwhile agreement".
"There are narrow but important gaps, and it's very important that any agreement is thorough, detailed and it's an agreement in which the whole world can have confidence," he said.
Mr Araqchi said "98% of the draft" had been agreed but that Tehran wanted a "right to enrichment" to be clearly stated in any deal.
"We are insisting on our right to enrichment, which should be clearly recognised in the draft agreement," Mr Araqchi was quoted as telling Iranian reporters late on Saturday - the fourth day of the negotiations.
EU foreign policy chief Baroness Catherine Ashton is leading the conference.
On Friday she briefly met Iranian Foreign Minister Mohammed Javad Zarif for a conversation that Iran's official Irna news agency described as "complicated and tough".
The Geneva meeting follows a previous round of talks earlier this month.
On that occasion, too, foreign ministers flew to Geneva to conclude the negotiations, but they went home empty-handed.
Analysts say a major sticking point is Iran's insistence on its right to enrich uranium - a process that yields material used to manufacture fuel for power stations, but can also be used in weapons.
Western diplomats are also concerned about a reactor Iran is building at Arak - an issue which disrupted the first round of talks.
US President Barack Obama has said any interim agreement would see the bulk of international and US sanctions remain, but that Iran would get sanctions relief worth between $6bn and $7bn.
The essence of the deal would involve Iran making no more advances in its nuclear programme and agreeing to "more vigorous inspections", he said.
Regional powers - notably Israel and Saudi Arabia - have been increasingly concerned at the prospect of an agreement with Iran.
Saudi Arabia has expressed disquiet at Washington's readiness to negotiate with Tehran.
"Appeasement hasn't worked in the past, and I don't think it will work in the 21st Century," the Saudi Ambassador to London, Prince Mohammed bin Nawaf bin Abdulaziz, said in an interview with Saturday's Times.
There have been persistent reports that Saudi Arabia is in a position to obtain nuclear weapons from Pakistan should Iran develop the bomb.
Pakistan has described such reports as "baseless" while Saudi Arabia points out that it is a signatory to the Non-Proliferation Treaty and wants a nuclear-free Middle East.
However, Prince Mohammed told the Times: "We are not going to sit idly by and receive a threat there and not think seriously how we can best defend our country and our region."