Iran nuclear talks enter their critical end-game Sunday with US Secretary of State John Kerry due to meet his Iranian counterpart in Switzerland as a March 31 deadline looms and a political storm rages in Washington.

After 18 months of talks and two missed deadlines, six world powers want by the end of the month to nail down the outline of a deal with Iran that would put making a nuclear bomb out of Tehran's reach.

A full accord is due by July 1. But as time runs short tempers have been boiling over in the United States, putting Kerry and US President Barack Obama under immense pressure.

Iran's supreme leader, Ayatollah Ali Khamenei, has meanwhile criticised the two-step process, saying matters should be handled in one fell swoop. He is due to give a closely watched Iranian New Year's address on March 21.

On Thursday Khamenei said the other side in the talks is "deceitful and stabs in the back," according to news agency ISNA.

From Lausanne, Iranian Foreign Minister Mohammad Javad Zarif is due to go to Brussels on Monday to meet his French, German and British counterparts before returning to Switzerland, according to his ministry.

- Republican letter -

On March 9, 47 Republican US senators wrote an open letter to Iran's leaders warning that any nuclear deal could be modified by Congress or revoked "with the stroke of a pen" by Obama's successors.

This followed an address to US lawmakers -- on a Republican invitation -- by Israeli Prime Minister Benjamin Netanyahu when he warned that the mooted deal would leave Iran's nuclear programme "largely intact".

Obama said that it was "somewhat ironic to see some members of Congress wanting to make common cause with hardliners in Iran". Kerry called the missive "irresponsible".

And German Foreign Minister Frank-Walter Steinmeier, whose country is one of the six powers in the talks, said Iran was increasingly sceptical as to "whether we are really serious".

Zarif, who together with reformist President Hassan Rouhani is also under pressure from their own hardliners, was scathing about the letter from US "extremists", saying it had "no legal value".

"In truth, it told us that we cannot trust the United States," Zarif told top clerics.

- Deal at hand? -

The deal being sought would involve Iran agreeing to shrink its activities to extend to at least a year the "breakout time" that Iran would, in theory, need to assemble a bomb's worth of fissile material.

Iran, saying its nuclear activities are purely for peaceful purposes, wants in return the lifting of painful UN and Western sanctions imposed over the last decade.

In November 2013, Iran and the "P5+1" powers -- the US, Russia, China, Britain, France and Germany -- agreed an interim accord involving a partial nuclear freeze by Iran and minor sanctions relief.

After numerous rounds of talks since, negotiators twice failed to turn this into a lasting deal by two self-imposed deadlines of July and then November of 2014, extending instead the interim arrangements.

Diplomats say there has been progress in some areas, including changing the design of the Arak reactor that could give Iran weapons-grade plutonium once operational.

EU foreign policy chief Federica Mogherini said on March 6 a "good deal is at hand".

But the key issue of the future scale of Iran's uranium enrichment capacities -- producing nuclear fuel but also potentially bomb material -- remains unresolved.

Other thorny topics involve the pace at which sanctions would be suspended and eventually lifted and the duration of the mooted accord. France believes the 10 years suggested in some quarters is not enough.

"If we cannot verify that they are not going to obtain a nuclear weapon, that there's a breakout period so that even if they cheated we would be able to have enough time to take action -- if we don't have that kind of deal, then we're not going to take it," Obama said in a TV interview aired on March 8.

What an Iran nuclear deal could look like
Geneva (AFP) March 13, 2015 - US Secretary of State John Kerry and his Iranian counterpart are due to meet in Switzerland on Sunday, seeking to agree the outlines of a nuclear deal by a March 31 deadline.

Few insights into the comprehensive accord under discussion since November 2013 have leaked, with officials remaining tight-lipped to protect the high stakes in the negotiations.

But US officials have laid out some of what they called the "bottom lines" of any deal, though without going into specifics.

Here are some of the possible contours of a deal:

The goal:

To reach a verifiable comprehensive agreement that limits Iran's ability to harness enough fissile material to build a nuclear bomb. In return, the international community would initially ease and then lift all sanctions imposed on the Islamic republic.

The negotiations have been extended twice but all sides say this will be the last round, having set a March 31 deadline for a political deal.

A final accord has to follow by the end of June.

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, which US officials have said they want to be in "double-digits".

Many assume that this figure is pointing to a deal lasting about 10 years, but US officials have refused to comment. France believes 10 years is not enough.

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.

According to documents leaked by the Israelis, and deemed accurate by non-proliferation experts, the US wants Tehran to reduce its total number of centrifuges to between 6,500-7,000.

Under the 2013 interim deal, Iran has halted production of 20 percent highly enriched uranium and eliminated or diluted much of its stock down to just five percent in return for limited sanctions relief.

Negotiators now seek to enshrine that agreement and cut Iran's stockpile of low-enriched uranium gas. A senior US official said Tehran would have to "significantly reduce" both the number of centrifuges and its uranium stocks.

There may also be a proposal allowing Iran to ship its uranium gas to Russia, which would convert it to fuel rods for the Bushehr nuclear plant which Moscow helped build.

Research and development:

Certain Western negotiators say that the limits on highly enriched uranium mean nothing if the agreement does not take into account the technological progress made by Iran.

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.

Monitoring:

A tough inspection programme using UN watchdog the IAEA is a cornerstone of any deal to ally fears that Iran could covertly develop a nuclear arsenal. 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 and suspect sites.

Sanctions:

Iran 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 -- could in fact be proving one of the most difficult tasks.