The United States and Iran said Monday they had made progress in the latest round of talks on Tehran's nuclear programme, but warned there was still a long way to go to seal a final deal.

Negotiators for Iran and six world powers had been meeting in Geneva since Friday, and plan further talks in Switzerland next week, a senior US administration official said.

"These were serious, useful and constructive discussions," the official said after US Secretary of State John Kerry and Iranian Foreign Minister Mohammad Javad Zarif wrapped up two days of meetings in the lakeside city.

Zarif agreed. "Some progress was made on certain subjects, but there is still a long road ahead to reach a final agreement," Iranian media quoted him as saying.

As a March 31 deadline looms for reaching a political framework for a deal, State Department spokeswoman Jen Psaki told reporters in Washington that Kerry "could certainly participate at some point" in next week's negotiations, but she had nothing concrete to announce.

"These talks have been productive, there's still more work to do," Psaki insisted.

The so-called P5+1 group of Britain, China, France, Russia, the United States and Germany are trying to strike an accord that would prevent Tehran from developing a nuclear bomb.

In return, the West would ease punishing sanctions imposed on Tehran over its nuclear programme, which Iran insists is purely civilian in nature.

- Won't be rushed -

Tortuous nuclear negotiations have poisoned relations between Iran and the West for years. However, there is now a heightened sense of urgency ahead of the March 31 deadline, with a second June 30 date by which to agree all the technical details of a final comprehensive accord.

"We all feel the pressure, no doubt about it, but that doesn't mean that it will make us rush to an agreement that does not fulfil the (US) objectives," the senior US official said in Geneva.

The goal was to "ensure that Iran does not acquire a nuclear weapon. That objective has to be met."

Tehran has voiced unhappiness with the separation of the political and technical aspects.

"We won't have a two-stage deal," said deputy foreign minister Abbas Araghchi, who is scheduled to meet Yukiya Amano, the head of the International Atomic Energy Agency, in Vienna Tuesday.

Zarif is scheduled to address the UN Human Rights Council in Geneva next Monday, fueling speculation he might combine the trip with further talks with Kerry.

In a sign of the growing push for an accord, US Energy Secretary Ernest Moniz took part in the talks for the first time, as did Ali Akbar Salehi, the director of the Iranian Atomic Energy Organisation.

Observers suggested their participation could be a sign a deal is within reach.

"It was necessary and appropriate to have technical people sit with Iran's technical people at the highest level in order to try to resolve any differences that may exist," Psaki said, adding it had not yet been determined if Moniz would return to Geneva next week.

Ahead of the talks, Kerry had warned "significant gaps" remained, and that after two previously missed deadlines, US President Barack Obama had "no inclination whatsoever to extend these talks beyond the period that has been set out."

- Very close -

White House spokesman Josh Earnest Monday also repeated assertions that the chances of a deal were "50-50 at best."

"Both sides are trying to reduce expectations, but there are also signs that usually only come towards the end of negotiations," said Trita Parsi, president of the National Iranian American Council.

"You usually don't threaten to walk away from the talks, as Kerry has, until the last phases of a negotiation, for insurance. Paradoxically, it could be a sign that they're very close," Parsi told AFP.

A key stumbling block in any final deal is thought to be the amount of uranium Iran would be allowed to enrich, and the number and type of centrifuges Tehran can retain.

Meanwhile, a 2012 claim by Israeli Prime Minister Benjamin Netanyahu that Iran was a year away from making a nuclear bomb was contradicted by his own secret services, according to leaked documents quoted Monday.

Just weeks after Netanyahu's dramatic speech to the UN, Mossad shared a report with South African intelligence which concluded Iran was "not performing the activity necessary to produce weapons," according to the British daily, the Guardian.

What an Iran nuclear deal could look like
Washington (AFP) Feb 24, 2015 - With the clock ticking down to a third, perhaps final deadline for a deal to rein in Iran's nuclear program, top diplomats are ramping up efforts to pin down details.

US Secretary of State John Kerry met twice in Geneva with Iranian Foreign Minister Mohammad Javad Zarif on Sunday and Monday, and could join fresh talks planned next week in Switzerland, US officials said.

"We're in the endgame now," Joe Cirincione, president of the Ploughshares Fund, who has long worked on non-proliferation issues, told AFP.

Few specific details of the accord under discussion since the November 2013 interim deal have leaked out, with officials remaining tightlipped to protect the integrity of the high-stakes negotiations.

But here are some of the key issues at stake:

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.

BREAKOUT TIME:

The US government and observers 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 fuel 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." But the exact timespan remains unknown and contentious.

ENRICHMENT:

This is one of the trickiest issues. 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 to 7,000.

Most experts say Iran would never agree to dismantle all its centrifuges.

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.

Negotiators now seek to enshrine that agreement and cut Iran's stockpile of low-enriched uranium gas.

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

NUCLEAR PLANTS:

The P5+1 wants to remove all uranium enrichment from Fordo, Iran's second main enrichment site deep in a bombproof bunker under a mountain near Qom, and concentrate it at a more accessible plant in Natanz.

On the unfinished heavy water reactor of Arak, there "is every indication they have arrived at a configuration that will result in it producing dramatically less plutonium than the original design," said Cirincione.

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

MONITORING:

A tough inspection program using the UN watchdog, the IAEA, is a cornerstone of any deal to ally any fears that Iran could covertly develop a nuclear arsenal. Under the interim deal, Iran has accepted daily inspections.

SANCTIONS:

Iran wants all sanctions, imposed by the US, EU and UN, 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 terror activities -- could in fact be proving one of the most difficult tasks.

CIVILIAN NUCLEAR PROGRAM:

Iran has always denied seeking a bomb, saying its nuclear program is for peaceful civilian energy purposes.

Under the Joint Plan of Action agreed in November 2013, the global powers known as the P5+1 acknowledged that a comprehensive deal "would enable Iran to fully enjoy its right to nuclear energy for peaceful purposes" in line with the nuclear Non-Proliferation Treaty (NPT).

Sources: Arms Control Association, the P5+1 Joint Plan of Action, Ploughshares Fund.