Iran and world powers have given themselves seven more months to reach a nuclear deal, but a heightened risk of hardliners wrecking the process means they may have missed their best chance ever, analysts say.

It also remains far from certain that putting yet more time on the clock will help, when after 10 rounds of talks the two sides still remain poles apart on the fundamental issues.

US Secretary of State John Kerry admitted as much on Monday after he and foreign ministers from Iran and the other powers decided to extend their deadline for a deal to July 1, after intense days of talks in Vienna.

"These talks aren't going to suddenly get easier just because we extend them," Kerry told reporters. "They are tough. They have been tough and they are going to stay tough."

Kerry and his Iranian counterpart Mohammad Javad Zarif stressed that some progress has been made, with Zarif even suggesting that a deal was possible "much faster" than in seven months. They aim to have the outlines by March.

But it is clear that when it comes to the two main sticking points -- the future scale of Iran's nuclear programme and the pace of sanctions relief for Iran -- they remain perhaps as far away as ever.

The five permanent members of the UN Security Council plus Germany want Iran to dismantle large parts of its activities in order to make it impossible for Tehran to make nuclear weapons -- an ambition it denies.

But the Islamic republic wants to expand its activities, most notably its capacity to enrich uranium -- a process which makes reactor fuel but also, potentially, the core of a nuclear bomb.

"I promise the Iranian nation that those (enrichment) centrifuges will never stop working," President Hassan Rouhani vowed Monday, calling UN and Western sanctions "tyrannical".

"Extending is not going to make it easier," former US State Department official Mark Fitzpatrick, now at the International Institute for Strategic Studies in London, told AFP.

"The problem is that there are fundamental differences. Iran is not willing to accept what would be required (for a deal)," he said.

- 'Fools to walk away' -

So why have the army of officials, experts and diplomats, shattered after 10 rounds of talks -- chief US negotiator Wendy Sherman has made clear she is tiring of Viennese speciality Wiener Schnitzel -- bothered putting more time on the clock?

The reason is quite simple. The alternative -- a collapse of the talks and a return to the explosive situation before last November's interim deal -- is far worse. Kerry said the parties would be "fools to walk away".

Before the November 2013 accord kicked in, Iran was enriching uranium to close to weapons-grade, the West was applying more and more sanctions and the danger of Israeli and US military action was growing.

Now, key parts of Iran's programme are frozen, meaning that, unlike before, the country is no longer creeping ever closer to potentially creating a bomb. In return, it receives around $700 million every month as overseas assets are freed up. This will continue until July.

"If either side decides to walk away, the consequences will be catastrophic," Kelsey Davenport, analyst at the Arms Control Association, told AFP.

"As long as Iran's programme remains capped and the sanctions remain capped then both parties are better off," agreed Fitzpatrick.

"Seven months is longer than anyone expected, but the longer the better."

- Obama under pressure -

But the extension carries the risk that critics in both Iran and the United States might lose patience.

Israel -- the Middle East's sole if undeclared nuclear-armed state -- and Gulf states are also sceptical.

Meanwhile, hardliners are waiting in the wings to undermine Rouhani, who has little to show in terms of boosting the Iranian economy from his strategy of sealing a rapprochement with the West.

But the bigger danger is for his US counterpart Barack Obama, with opposition Republicans in control of both houses of Congress from January.

No sooner had the extension been announced Monday that several Republicans advocated more sanctions.

"Now, more than ever, it's critical that Congress enacts sanctions that give Iran's mullahs no choice but to dismantle their illicit nuclear program," said Senator Mark Kirk.

But more sanctions -- Obama's veto could be overridden if enough rebel Democrats support new sanctions -- would be a breach of last November's interim deal and could prompt the Iranians to walk away.

"We should give the negotiators as much time as they need to finish their work," said Ali Vaez from the International Crisis Group.

"Enormous progress has been made. This is now too big to fail."

US Congress skeptical of Iran nuclear talks extension
Washington (AFP) Nov 24, 2014 - US House and Senate members reacted coolly Monday to an extension of international negotiations over Iran's nuclear program, with skeptical lawmakers demanding Congress "tighten the economic vice on Tehran" through new sanctions.

No sooner had the extension been announced in Vienna and US Secretary of State John Kerry implored US lawmakers not to "walk away" from the negotiations by slapping punitive sanctions on Iran, that several lawmakers advocated just that, setting up a potential White House clash with Congress.

"Now more than ever, it's critical that Congress enacts sanctions that give Iran's mullahs no choice but to dismantle their illicit nuclear program," Republican Senator Mark Kirk, who supports new sanctions, said in a statement.

"Congress will not give Iran more time to build a nuclear bomb."

Kirk has drawn up tough legislation with Senate Foreign Relations Committee chairman Robert Menendez, a Democrat, that would see sanctions kick in should Iran violate terms of the temporary agreement or future deal.

But the Senate leadership has refused to bring it to the floor, allowing a chance for President Barack Obama's administration to pursue its negotiations.

Menendez suggested legislative action was vital to a breakthrough.

"I intend to work with my Senate colleagues in a bipartisan manner in the coming weeks to ensure that Iran comprehends that we will not ever permit it to become a threshold nuclear state," he said.

- New sanctions 'counterproductive' -

But Senator Tim Johnson, chairman of the Banking Committee through which sanctions legislation often passes before getting a full vote, warned that imposing new sanctions "now would be grossly counterproductive."

Hawkish Senators John McCain, Lindsey Graham and Kelly Ayotte said they supported additional sanctions as negotiations continue, and demanded "any final deal between Iran and the United States be sent to Congress for approval."

House Foreign Affairs Committee Chairman Ed Royce said Obama should support new sanctions as a way to increase leverage on Iran.

"This seven-month extension should be used to tighten the economic vice on Tehran -- already suffering from falling energy prices -- to force the concessions that Iran has been resisting," Royce said in a statement.

Royce was backed by panel Democrat Brad Sherman, who said new sanctions are the "best hope for a good agreement."

House Speaker John Boehner was more vague in urging pressure on Tehran.

"Instead of giving Iran more flexibility, we should be holding this regime accountable for the threat it poses to the region," Boehner said.

Senator Bob Corker, tipped to replace Menendez as the powerful Foreign Relations chairman when Republicans take full control of Congress in January, supported a threat of new sanctions should the Iran deal ultimately fall apart.

"Congress must have the opportunity to weigh in before implementation of any final agreement and begin preparing alternatives, including tougher sanctions, should negotiations fail," Corker.

The State Department acknowledged the role of sanctions in bringing Tehran to the negotiations.

"On the other hand, sanctions are not alone going to get us the comprehensive deal," State Department spokesman Jeffrey Rathke told reporters.