Foreign ministers from six world powers are meeting their Iranian counterpart, amid hopes of a breakthrough after almost 18 months of negotiations.

They want to impose limits that would prevent Iran from producing enough fuel for a nuclear weapon within a year.

Iran, which insists its nuclear programme is peaceful, wants to see crippling sanctions lifted in return.

Iranian and Western officials have said that a deal is possible, but that some issues are still to be resolved.

'No compromise'

The negotiations in Lausanne were expected to intensify on Monday, with foreign ministers from the so-called P5+1 - comprising the US, UK, France, China, Russia and Germany - meeting Iran's Mohammad Javad Zarif. The EU foreign policy chief Federica Mogherini is also there.

They are aiming to agree on a political framework by Tuesday night that would set the stage for talks on the technical details of a comprehensive accord, due by 30 June.

"We are here because we believe a deal can be done," UK Foreign Secretary Philip Hammond told reporters as he arrived in Lausanne on Sunday night.

"But it has to be a deal which puts the bomb beyond Iran's reach," he added. "There can't be any compromise about that."

At the scene: Lyse Doucet, BBC News, Lausanne

It's going down to the proverbial wire in this crucial round of nuclear talks. One Western diplomat spoke of "some quite important gaps".

Two main disputes continue to be which sanctions will be lifted and when, and what kind of nuclear research and development Iran can still pursue.

As Iran's team sits with all six foreign ministers, observers say it's still not clear if they're engaging in high stakes bargaining or have no more political space to move forward.

For Iran, elements like ending sanctions tied to UN Security Council resolutions matter in financial and political terms. For world powers, "putting the bomb beyond Iran's reach" is the main imperative.

All sides indicate talks could slip past Tuesday's self-imposed deadline, into the early hours of Wednesday. All sides use the word "tough" to describe this historic endgame. Even if they agree a framework, there's still a huge amount of work ahead to draft a final deal by the end of June.

(BBC)