The issues dividing Iran and the west at nuclear talks in Geneva have in some cases been whittled down to single words, but those words are important enough to keep an elusive deal just beyond reach, according to diplomats at the negotiations.

For the second time in little over two weeks, foreign ministers from seven countries have been holed up in a Geneva hotel in an attempt to resolve one of the most intractable problems of the early 21st century – the international standoff over Iran's nuclear programme. Despite constant assurances of progress and narrowing gaps, they were still struggling to strike a bargain as evening fell on a third day of talks.

Much of Saturday's negotiations involved the US secretary of state, John Kerry, and his Iranian counterpart, Mohammad Javad Zarif, in encounters chaired by the EU foreign policy high representative, Catherine Ashton. Kerry and Ashton then compared notes with foreign ministers from France, the UK, Germany, Russia and China, who flew to Geneva in the hope of closing a deal.

On arrival on Saturday morning, William Hague sought to damp down expectations that the ministers' arrival might mean an agreement was necessarily at hand. "The foreign ministers have come to support these negotiations and to be able to confer together easily and quickly if we need to make fresh decisions of any kind," Hague said. "They remain 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."

He added: "They are narrow gaps but they are important gaps, and it's very important that any agreement here is thorough, that it is detailed, that it is comprehensive, and that it's a deal in which the whole world can have confidence." He did not spell out the nature of the remaining gaps, but said: "They are the same areas of difficulty that we had two weeks ago, which should be a good clue, when we were last here in Geneva."

The focus of the negotiations is a text which lays out a plan for a short-term confidence-building agreement, intended to stop the clock on the build-up to a new conflict in the Middle East, which at the same time sketches the outline of long-term solution to the decade-old standoff over the Iranian programme.

Text in dispute has been put in brackets, and over the course of the past three days, diplomats said, that text had shrunk. "On some issues, it comes down to a single word, but what that word is and what it means is critically important. We have to get this absolutely right," said a western official.

Zarif described the talks in similar terms. "We have reached the point of writing and it's difficult because we are insisting on Iran's national interests and its rights and we don't want Iran's rights to get twisted," the Iranian foreign minister said. "This is why we are paying a great deal of attention to words and phrases that each have their own meanings. There will be no agreement until we agree on every single issue."

According to Iran's deputy foreign minister, Abbas Araqchi, primary among the remaining problems was the question the agreement should explicitly endorse Iran's right to enrich uranium. Western states are reluctant, fearful that it would create a precedent, prompting other countries to insist on enrichment, a grave proliferation risk. Enrichment can produce reactor fuel or the fissile core for a warhead.

As the talks stretched on into a fifth day, Aragchi described them as being "in their 11th hour". He said: "We have agreed to 98 percent of the draft ... but the remaining 2 percent is very important to us.

"The issue of enrichment should be written in the agreement and Iran's enrichment will be recognised. This is our most important concern in the talks."

Hague said that the narrowing of differences meant "there is a huge amount of agreement and it remains the case that there has been a huge amount of progress being made in recent weeks".

The role played by France and its foreign minister, Laurent Fabius, has come under particular scrutiny. The last round of Geneva talks broke up in the early morning of 10 November, after Fabius insisted on the last night of the talks on a toughening of the western position.

He angered his fellow diplomats on that occasion by breaking protocol, publicly discussing the content of the negotiations and French policy towards them, and by breaking the news of the failure to win agreement before Ashton and Zarif could issue a joint statement.

"I hope for an accord but it must be a solid deal. I'm here to work for that," Fabius said this weekend.