US Secretary of State John Kerry (R-seated) talks with members of his delegation after a meeting with Iranian Foreign Minister Javad Zarif and other officials at the Beau Rivage Palace Hotel March 27, 2015 in Lausanne, Switzerland

Lausanne (AFP) - Tough negotiations to end a decade-long standoff over Iran's suspect nuclear programme move into top gear Saturday, seeking to overcome stubborn hurdles as both sides urged painful compromises.

The stakes couldn't be higher. Global powers are chasing a deal to rein in Iran's nuclear ambitions which have poisoned international relations for 12 years and seal 18 months of closed-door, painstaking talks which have cris-crossed the globe and involved hundreds of experts.

French Foreign Minister Laurent Fabius, seen as the most hawkish in the so-called P5+1 group of Britain, China, France, Germany, Russia and the United States overseeing the talks, will be the first European minister to fly in for the crunch weekend in Lausanne, Switzerland.

He will join US Secretary of State John Kerry and Iran's new face of diplomacy Mohammad Javad Zarif, chasing an agreement on the broad outlines of a deal by a Tuesday deadline.

Since dragging talks resumed in earnest in 2013, Kerry and Zarif have met multiple times seeking to hammer out a deal which will seal off Iran's pathways to a nuclear weapon.

In return Iran, which has insisted its nuclear programme is only for civilian energy needs, wants an easing of a labyrinth of international sanctions which have excluded the Islamic republic from lucrative oil markets and crippled its economy.

But the road to a deal has been strewn with hurdles, and even with a deadline just days away, Iran and the US Friday were still insisting the other side had to make concessions.

- 'We have differences' -

"We have tough negotiations and we have differences and we have to solve them," Zarif told reporters.

The two sides remained at odds over an agreement, he said, adding: "No, we are not (close). But we are making progress."

"The work is very complicated and difficult. The other side needs to choose between pressure and a political accord," Zarif said.

A US State Department official, asking not to be named, agreed the negotiations "have been tough and very serious."

"We're at that point in the negotiations where we really need to see decisions being made," the official said.

World powers hope to scale back Iran's nuclear capacity by cutting its number of sophisticated centrifuges for spinning enriched uranium, which can be used to make a bomb, and reconfiguring some of its facilities.

Fabius will lead the charge to Lausanne, with Kerry agreeing with his Russian counterpart Sergei Lavrov that a joint P5+1 ministerial meeting could be possible on Sunday.

Chinese news agency Xinhua, quoting a source close to the Chinese delegation, confirmed China's Foreign Minister Wang Yi will join the talks on Sunday, with the EU's foreign policy chief Federica Mogherini also expected to joined the lakeside negotiations the same day.

British Foreign Secretary Philip Hammond said he was "hopeful that we're going to be making that progress over the next 48 hours" adding he would "jump on a plane and go to Lausanne as soon as we sense that we're getting to the point where we need to sit down."

In Washington, President Barack Obama and German Chancellor Angela Merkel also upped the pressure on Iran.

"Iran must make the decisions necessary to resolve several remaining issues," a statement said after the two leaders spoke by telephone.

Iran would have to make "painful choices," a Western diplomat echoed, saying its development of faster nuclear machinery, the possible military dimensions of its programme, and the lifting of sanctions were among the unresolved issues.

- Sanctions conundrum -

Iranian President Hassan Rouhani called Thursday for the "unjust" sanctions choking the country's economy to be lifted, with global powers insisting sanctions will only be suspended, not lifted, as a safeguard against any violations.

The emerging accord is to be rounded out with complex technical annexes by a June 30 deadline.

As to how the eventual deal might be presented, Hammond told reporters in Washington it would be a political outline of an agreement to be finalised over the coming months.

"Conceptually what's envisaged is a statement of broad understanding, the political parameters," he said. Other diplomats said there may just be some kind of declaration and the specific details might not be made public.

By AFP