With a deadline hours away, Iran and six world powers ramped up the pace on Tuesday in negotiations over a preliminary deal on Tehran's nuclear programme, while officials cautioned that any agreement would likely be fragile and incomplete.

For nearly a week, the United States, Britain, France, Germany, Russia and China have been trying to break an impasse in the talks, which are aimed at stopping Iran from gaining the capacity to develop a nuclear bomb in exchange for easing international sanctions that are crippling its economy.

But disagreements on enrichment research and the pace of lifting sanctions threatened to scupper a deal that could end a 12-year standoff between Iran and the West over Tehran's nuclear ambitions and reduce the risk of another Middle East war.

"The two sticking points are the duration and the lifting of sanctions," an Iranian official said. "The two sides are arguing about the content of the text. Generally progress has been made."

Officials played down expectations for the talks in the Swiss city of Lausanne.

For days they have been trying to agree on a brief document of several pages outlining key headline numbers to form the basis of a future agreement. Officials said they hoped to be able to announce something, though one Western diplomat said it would be "incomplete and kick some issues down the road".

Negotiations among the parties on sticking points went into the night and continued on Tuesday. They were expected to run late and possibly into Wednesday. Officials said they were hoping to agree on some kind of declaration, while any actual preliminary understanding that is agreed might remain confidential.

It was also possible they would not agree on anything.

"We are preparing for both scenarios," another Western diplomat said.

Officials said talks on a framework accord, intended as a prelude to a comprehensive agreement by the end of June, could yet fall apart.

"There still remain some difficult issues," US Secretary of State John Kerry told CNN. "We are working very hard to work those through. We are working into the night."

STICKING POINTS

French Foreign Minister Laurent Fabius and his German counterpart Frank-Walter Steinmeier cancelled plans to go to Berlin for a French-German summit on Tuesday. "The negotiations are at a critical and difficult phase, making the presence of both ministers in Lausanne essential," a German government source said.

The real deadline in the talks, Western and Iranian officials said, is not Tuesday but June 30.

They said the main sticking points remain the removal of U.N. sanctions and Iranian demands for the right to unfettered research and development into advanced nuclear centrifuges after the first 10 years of the agreement expires.

Iran said the key issue was lifting sanctions quickly.

"There will be no agreement if the sanctions issue cannot be resolved," Majid Takhteravanchi, an Iranian negotiator, told Iran's Fars news agency. "This issue is very important for us."

The six powers want more than a 10-year suspension of Iran's most sensitive nuclear work. Tehran, which denies it is trying to develop a nuclear weapons capability, demands a swift end to sanctions in exchange for temporary limits on its atomic activities.

Russian Foreign Minister Sergei Lavrov, who was due back in Lausanne in the afternoon, told reporters in Moscow he believed there was a good chance of success.

"The chances are high. They are probably not 100 percent but you can never be 100 percent certain of anything. The odds are quite 'doable' if none of the parties raise the stakes at the last minute, he said.

Both Iran and the six have floated compromise proposals, but Western officials said Tehran has recently backed away from proposals it previously indicated it could accept, such as on shipping enriched uranium stocks to Russia. Officials close to the talks said dilution of the stockpiled uranium was an option, noting that the stockpiles issue was not a dealbreaker.

The goal of the negotiations is to find a way to ensure that for at least the next 10 years Iran is at least one year away from being able to produce enough fissile material for an atomic weapon. In exchange for temporary limits on its most sensitive atomic activities, Tehran wants an end to sanctions.

Iran and the six powers have twice extended their deadline for a long-term agreement, after reaching an interim accord in November 2013.

With the US Congress warning it will consider imposing new US sanctions on Iran if there is no agreement this week, there is a sense of urgency in the talks.

"With Congress, the Iranian hawks and a Middle East situation where nobody's exactly getting on, I'm not convinced we'll get a second chance if this fails," the Western diplomat said.

US President Barack Obama has threatened to veto any sanctions moves by the Republican-dominated Congress. – Reuters