Global powers were due to gather with Iran Monday, seeking to slot into place the last pieces of a complex puzzle to curtail Tehran's nuclear programme as diplomats said tentative agreement on some parts had been reached.

US Secretary of State John Kerry will lead the team of six world powers -- who hope an end to more than a decade of nuclear tensions with Iran may be in sight -- for their first full plenary session of the latest round of talks.

While diplomats said some key points appeared to have been resolved to ensure Iran cannot make a covert dash for a nuclear bomb, they cautioned that the outlines of a political understanding were not yet fully agreed.

"We are here because we believe a deal can be done," Hammond told reporters in Lausanne late Sunday as he became the last of the P5+1 group of foreign ministers to arrive in the Swiss town for the talks.

"But it has to be a deal which puts the bomb beyond Iran's reach," he said, adding he hoped for success before Tuesday's midnight deadline.

"There can't be any compromise about that," Hammond insisted, before meeting with his Chinese, French, German, Russian and US counterparts, as well as EU foreign policy chief Federica Mogherini.

As negotiators in Switzerland raced against the clock, Israeli Prime Minister Benjamin Netanyahu launched a blistering attack on the "dangerous" deal.

- Guarded optimism -

One Western diplomat said Iran had "more or less" agreed to slash the number of its sophisticated centrifuges from 20,000 to 6,000 and to ship abroad most of its stockpile of nuclear material.

But Iran's chief negotiator Abbas Araqchi insisted: "There is no question of sending the stocks abroad."

Iranians have expressed guarded optimism that after 18 months of tortuous negotiations and two missed deadlines, a breakthrough may be at hand.

"Getting to an accord is doable. Solutions have been found for numerous questions. We are still working on two or three issues... The talks are in their final phase and are very difficult," Araqchi said.

The aim is to agree broad outlines for an accord by midnight on Tuesday and then flesh out a series of complex annexes containing all the technical details by June 30.

The mooted deal would see Iran scale down its nuclear programme and allow unprecedented inspections of its remaining activities.

Its underground facility at Fordo would also likely be barred from uranium enrichment, diplomats said.

One of the thorniest issues is the lifting of UN, EU and US sanctions that have choked Iran's economy by strangling its oil exports and banks.

Araqchi said Sunday there must be a "precise framework" for lifting sanctions. The duration of any deal -- the US wants at least 10 and possibly up to 15 years -- is also a point of contention.

"There are six (UN Security Council) resolutions that have to be annulled," Araqchi said.

But a senior US official said all sides had agreed to a phased, step-by-step approach, although the exact formula had yet to be found.

- 'Dangerous axis' -

For Israel and US President Barack Obama's Republican foes however, the agreement will not go far enough to stop Iran one day getting the bomb.

"The dangerous accord which is being negotiated in Lausanne confirms our concerns and even worse," Netanyahu said in remarks broadcast on public radio.

The "Iran-Lausanne-Yemen axis" was "dangerous for all of humanity", he said.

Republicans are also getting ready to pounce, planning new sanctions against Iran.

"I just don't understand why we would sign an agreement with a group of people who in my opinion have no intention of keeping their word," US House Speaker John Boehner told CNN.

What an Iran nuclear deal could look like
Lausanne (AFP) March 29, 2015 - Global powers were Sunday narrowing in on a deal to stop any Iranian bid to develop nuclear weapons, with diplomats saying some key parts of an outline had been tentatively agreed.

The "framework" accord -- no one knows how detailed it will be -- is meant to be fleshed out into a comprehensive agreement by June 30 to end over a decade of tensions with the Islamic republic.

Here are the possible contours of such an agreement, which Iran and the so-called P5+1 -- Britain, China, France, Germany, Russia, and the United States -- have been negotiating since late 2013.

The goal:

To reach a verifiable comprehensive agreement that limits Iran's ability to harness enough fissile material to build a nuclear bomb.

Breakout time:

World powers want to cut Iran's ability to build an atomic weapon to a "one-year breakout time." That would mean Tehran would need at least 12 months to be able to produce enough material for a nuclear bomb.

The assumption is that the international community would have enough time to detect such a move -- and could seek to strike or destroy the facilities.

This year-long breakout time would stay in place for the length of the deal. Officials said Sunday that they were homing in on a 15-year duration for any deal, but stressed that different elements would be subject to a variety of deadlines.

Enrichment:

This is one of the trickiest issues. Several years ago, the international community wanted to deny Iran any capability to enrich uranium.

In April 2006, Iran launched a process to enrich uranium to 3.5 percent. By February 2010, Iran had the ability to enrich to 20 percent, giving it the possibility of moving quickly to 90 percent -- the level needed for a bomb.

Iran currently has about 19,000 centrifuges. About 10,200 centrifuges are in operation, used for spinning uranium gas at supersonic speeds to make it suitable for power generation and medical uses but also, at high purities, for a bomb.

Diplomats told AFP there had been tentative agreement that Iran would slash the total number of its centrifuges by about two-thirds, to about 6,000.

Sanctions:

Iran has publicly said it wants all sanctions imposed by the US, European Union and United Nations to be lifted. But world powers have refused, talking instead about a phased, gradual easing of the measures. Experts say untangling the sanctions -- from those also imposed for Tehran's alleged terror activities for example -- was proving to be one of the most difficult tasks.

A senior US official said all parties, including Iran, had agreed there should be a phased, step-by-step, reciprocal approach. There is still disagreement on the formula to be adopted.

Sanctions relief would be staggered and linked to certain "milestones" by Iran over the lifetime of the deal.

Research and development:

Certain Western negotiators say the limits on highly enriched uranium mean nothing if the agreement does not take into account the technological progress made by Iran. A US official said Sunday research and development remained one of the biggest areas of contention.

Iran's nuclear chief Ali Akbar Salehi has said the deal would not stop Iran "continuing with force" the development of more powerful and modern centrifuges.

Nuclear plants:

Any deal will have to lay out what nuclear sites Iran would be allowed to maintain. The US does not want Iran to be allowed to develop weapons-grade plutonium at its unfinished Arak reactor.

Plutonium can be used as an alternative fissile material to highly-enriched uranium.

Iran should also not use its Fordo nuclear plant to enrich uranium, which would leave only its Natanz plant capable of enriching uranium.

Diplomats have told AFP that the virtually impregnable underground Fordo facility near the holy city of Qom would cease uranium enrichment, hinting it would remain open for other purposes such as medical research.

Monitoring:

A tough inspection programme using UN watchdog the IAEA, the International Atomic Energy Agency, is a cornerstone of any deal. The US bottom line is that Iran must agree to unprecedented inspections of both nuclear and production facilities as well as uranium mines and mills.

Getting a clean bill of health from the IAEA depends in large part on movement in its stalled probe into allegations, mostly before 2003, that Iran's programme had "possible military dimensions".

Statement or declaration?

US officials say they have not decided what form any announcement would take, whether it will be a written statement or just a declaration. But they concede some of the top lines of any understanding would have to be publicly communicated.