A deal over Iran's nuclear programme will only be agreed if it is "truly worthwhile", Foreign Secretary William Hague said today as he arrived in Geneva for international talks.
Mr Hague joined counterparts including US secretary of state John Kerry at the meeting and the presence of senior ministers has raised expectations that a deal could be close.
Negotiators have been working since Wednesday to find language acceptable to Tehran and the E3+3 group - the US, Russia, China, France, Britain and Germany.
Arriving at the talks, Mr Hague told reporters: "We have come now to support these negotiations and be able to confer together easily and quickly if we need to make fresh decisions of any kind.
"They remain very difficult negotiations, I think it's important to stress that. We're not here because things are necessarily finished, we're here because they're difficult and they remain difficult.
"There are narrow gaps but they are important gaps. It's very important that any agreement is thorough, that it is detailed, that it is comprehensive, and that it is a deal in which we can all - the whole world - have confidence that it can work and it will be observed."
He added: "We will only make a deal - the six countries involved - will only make an agreement if we think it is a truly worthwhile agreement and really does address the problems caused by Iran's nuclear programme."
Mr Hague said a "huge amount of progress" had been made in recent weeks but some "difficult areas" remained from talks in Geneva two weeks ago.
He added: "There is a huge amount of agreement and it remains the case that a huge amount of progress has been made in recent weeks. The state of this negotiation is entirely different from a few months ago."
Mr Hague said he understood the concerns of Saudi Arabia after its ambassador to London, Prince Mohammed bin Nawaf bin Abdulaziz, vented frustration at the West's dealings with Iran in an interview with the Times newspaper.
"It's understandable people have concerns," Mr Hague told reporters.
"Much of the recent history in recent years on Iran's nuclear programme is of concealment and is in defiance of international agreements and resolutions.
"That is why it's important that any agreement is thorough and detailed and comprehensive, and covers all aspects of Iran's nuclear programme."
A sticking point in the talks has been Tehran's claim to a right to produce nuclear fuel and Iranian foreign minister Mohammad Javad Zarif and Baroness Ashton, the European Union's top diplomat, have met repeatedly since Wednesday trying to resolve that and other differences.
Mr Zarif and Baroness Ashton met briefly for talks that Iran's official Irna news agency described as "complicated and tough".
It quoted Iranian deputy foreign minister Abbas Araghchi in Geneva as saying that Iran's right to uranium enrichment must be part of any deal. Iran claims it is enriching only for reactor fuel, medical uses and research. But the technology can also produce material for a nuclear warhead.
Mr Zarif last weekend indicated that Iran was ready to sign a deal that does not expressly state Iran's right to enrich, raising hopes that a deal could be sealed at the current Geneva round.
He was slapped down by Iran's supreme leader, Ayatollah Ali Khamenei, who said his country would never compromise on "red lines".
But it has been reported that officials have been working on a compromise avoiding a direct reference to any country's right to enrich but still giving enough leeway for Iran to accept it.
A previous round of talks attended by foreign ministers including Mr Hague, Mr Kerry, Russia's Sergey Lavrov and senior counterparts earlier this month failed to achieve a breakthrough, with reports suggesting France was holding out for tougher conditions to be placed on Iran in return for the possible lifting of some sanctions.
Relations between Tehran and the West have thawed since the election of president Hassan Rouhani earlier this year.
In recent days David Cameron became the first British Prime Minister to call an Iranian president in more than a decade, with Downing Street saying both leaders agreed it was "important to seize the opportunity" presented by the talks.
Mr Kerry and Mr Hague will have talks in London tomorrow on the Middle East, the US State Department said.
As well as further discussions on Iran, they are expected to consider the latest developments in Syria and the Middle East peace process.