Iran is "forever" banned from building a nuclear weapon under an existing international treaty, top US diplomat John Kerry said Wednesday, seeking to dismiss fears that limits on its program imposed in a new deal may eventually be lifted.

"Please understand, there is no reduction (of restrictions) at any time that permits Iran to build a nuclear weapon," Kerry told US lawmakers when pressed whether a deal with Tehran would eventually allow them to develop atomic arms.

"Iran is forever forbidden from building a nuclear weapon, that is the nature of membership in the Non-Proliferation Treaty which they are a member of."

Iran is a signatory of the nuclear NPT, but has already been found in non-compliance with the global treaty, including for failing to declare its uranium enrichment program.

World powers -- Britain, China, France, Germany, Russia and the United States -- appear to be close to nailing down a deal with Iran which would rein in its suspect nuclear program.

The aim is to construct a deal, which through limiting enrichment and destroying centrifuges for example, would ensure that it would take the Islamic Republic at least a year to gather together the fissile material to build a bomb. That would give the international community ample time to ward off the threat.

US officials have said they want the deal to remain in place for an amount of time spanning "double digits" but denied they were thinking of only 10 years.

At a hearing of the House Foreign Affairs Committee, Kerry was seeking to dismiss reports that after that time Iran would be free to develop atomic weapons.

But he stressed that "Iran has already mastered the fuel cycle" and insisted that the key was to ensure that its nuclear program "was exclusively for peaceful purposes" -- as allowed for under the NPT.

US and Iranian negotiators are due to meet again next week in Switzerland, and American officials have said Kerry could again join the talks as a March 31 deadline for a political framework on the Iran deal looms.

National Security Advisor Susan Rice said that any deal would be a "phased one."

Iran is "not going to be able to convince anybody on day one that they have stopped enrichment," she told PBS television late Tuesday.

"They're going to have to prove over time through their actions which will be validated that they are, in fact, upholding their commitments. So this will be a phased process any way you slice it."

Rouhani seeks clergy support in Iran's holy city
Tehran (AFP) Feb 25, 2015 - Iranian President Hassan Rouhani, as decision time looms for a nuclear deal with world powers, sought Wednesday to win the support of the religious community at the "backbone" of the Islamic republic.

"For the government and the people of Iran, Qom is not a city, but the symbol of religious life," Rouhani, who faces criticism on the home front, said in a speech in the Shiite holy city of Qom.

At the heart of the Shia faith in Iran, Qom's seminary hosts about 80,000 religious students, some from abroad.

"I want to make clear that the government needs Qom" with its clergy forming the "backbone" of Iran, he said, stressing the seminary's independence would "never be compromised under the banner of a policy, a party or faction."

Rouhani, himself a middle-ranking clergy, is often criticised by ultraconservatives over his cultural and political openness, as well as being accused of making too many concessions in nuclear talks with world powers.

The timing of his Qom visit, as nuclear negotiations enter the final stage, "can be seen as an effort to gain the support of religious leaders for a possible agreement", reformist daily Shargh said.

It said Rouhani, a reputed moderate, "knows that having the consent of religious leaders from Qom can help him push forward his policies and keep his election promises".

EU foreign policy chief Federica Mogherini said Tuesday that a deal between Tehran and world powers on Iran's controversial nuclear programme was "at hand", ahead of a new round of talks scheduled in Geneva next week.

They have a March 31 deadline looms reaching a political framework for a deal.

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 program, which Iran insists is purely civilian.