Britain's Foreign Secretary William Hague has arrived for talks in Geneva amid hopes an historic deal may be signed on Iran's nuclear programme.
Mr Hague joined counterparts including US Secretary of State John Kerry and foreign ministers from Russia, France and Germany, and the presence of senior ministers has raised expectations that a deal could be close.
Arriving at the talks, he told reporters: "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."
Earlier, Iranian officials said "two or three" differences remain as talks aimed at curbing Tehran's nuclear ambitions go into a fourth day.
The arrival of Russia's foreign minister Sergei Lavrov had heightened speculation that US Secretary of State John Kerry would also attend.
John Kerry has also joined the talks in Geneva Mr Kerry's goal is to "help narrow differences and move closer to an agreement," a State Department spokeswoman said.
Mr Lavrov joined the talks as negotiators said there had been some progress on the third day of meetings and the White House said the US remained "hopeful" that agreement could be reached.
This third meeting since President Hassan Rouhani's election in June is seen as the biggest hope in years to resolve the decade-old stand-off over Iran's nuclear programme.
Failure might mean Iran resuming the expansion of its atomic activities, while Washington and others could toughen already painful sanctions and the possibility of Israeli military action would draw nearer.
At the last gathering, foreign ministers including Mr Kerry flew to Geneva but three days of intense talks failed and they went home empty-handed.
Sky's Foreign Affairs Editor Tim Marshall, in Geneva, said: "They've spent the last 12 days talking to each other via the phone at political director level and I think they've got so close.
Iran's Foreign Minister said talks were 'progressing well' on Friday "They're burning political capital here, the foreign ministers. If they come in again and leave empty handed again, they've burned a lot of capital and the voices that say 'what is the point of this' will grow ever louder.
"And I think if they don't get a deal, you might see extra sanctions coming from the US Congress next week and that will scupper the whole deal."
According to a draft proposal hammered out on November 9, the US, Britain, China, France, Russia, and Germany - the so-called P5+1 nations - want Iran to freeze key parts of its nuclear programme for six months.
In return Iran would get minor and, Western officials insist, "reversible" sanctions relief, including unlocking several billion dollars in oil revenues and easing trade restrictions on precious metals and aircraft parts.
This hoped-for "first phase" deal would build trust and ease tensions while negotiators push on for a final accord that ends once and for all fears that Tehran will get an atomic bomb.
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