Freed from the constraints of a lengthy election campaign, US President Barack Obama now has another shot at pressing Iran through diplomacy to scale back its nuclear programme, analysts say.

But despite Obama's increased leeway and tough sanctions against Iran, resolving the decade-old deadlock and silencing Israel's "drums of war" remains a tall order indeed.

A new round of talks between Iran and six world powers, the first since June, is expected by the end of the year, or at the latest in early 2013, analysts say. A rare bilateral US-Iran meeting might even happen.

"It's pretty clear that the United States and its European allies are gearing up to try again for diplomatic engagement," Mark Fitzpatrick, nuclear expert at the International Institute for Strategic Studies in London, told AFP.

"But the question is, what will be on the table? Iran won't be making concessions unless it gets some form of sanctions relief."

The last offer from the P5+1 -- the US, China, Russia, Britain, France and Germany -- was made to Iran in May by the group's chief negotiator, EU foreign policy chief Catherine Ashton, in high-level talks in Baghdad.

This "Stop, Shut and Ship" package called on Iran to cease enriching uranium to purities of 20 percent -- technically speaking not far short of the 90 percent needed for a nuclear weapon. It also told Iran to close its Fordo enrichment facility, and to export its existing stockpiles of 20-percent uranium.

Iran, which insists its nuclear programme is peaceful, rejected the proposals as they failed to offer sufficient relief from sanctions that have begun to cause real problems for its economy, while insisting on a recognition of its "right" to enrich.

Since another P5+1 meeting in Moscow in June, the process -- as well as parallel talks between Iran and the UN atomic agency, the IAEA -- has been on ice because of the US election campaign.

"The administration was in a very defensive position for the past six months," Mark Hibbs from the Carnegie Endowment for International Peace said. "It was also difficult for Iran because they didn't want to negotiate with someone who might not be in office after November."

-- NO PAIN NO GAIN --

According to Siavush Randjbar-Daemi, a Middle East and Iran lecturer at Manchester University in England, Iran has been making "smoke signals" that it wants to talk.

A major reason for this, he believes -- though other analysts are not so sure -- is that what Tehran calls the "economic war" waged against it by the West in the form of sanctions is making the regime more pliable.

The sharp drop in oil revenues "is not sustainable in the long run," Randjbar-Daemi told AFP. "I don't think Iran realistically has any other option than seeking an understanding with the West."

But in return the Iranian leadership won't want to lose face, meaning the P5+1 will have to sweeten their package of proposals.

This means first and foremost swift sanctions relief, which is easier said than done, particularly for Obama.

"Obama may only have the authority to waive a very limited degree of sanctions," said Trita Parsi, the Iranian-born author of a book about Obama's diplomacy early in his first term when the president famously offered Iran an "extended hand."

"Congress ultimately holds the power to lift sanctions, and there is a near consensus on Capitol Hill that it will not happen anytime soon."

In any case, a vicious circle exists because the P5+1 will insist on "confidence-building measures" from Iran before promising any let-up in economic pressure.

"There is reason for some optimism, but it is guarded optimism because in the final analysis it depends on whether Iran will 'play'," said Hibbs. "If they won't, all bets are off."

-- RED LINE --

Parsi believes Obama's "unique opportunity ... to make diplomatic headway" will expire in mid-March, when campaigning starts for Iran's presidential election in June.

"After that, Iran enters its own election season and the paralysis that comes with it. This may be his last best shot to resolve the US-Iran conflict peacefully."

Time is also of the essence because every day Iran is enriching more uranium and expanding its nuclear facilities, as outlined in the IAEA's quarterly reports (the next is due this month).

This fact is not lost on Israel, the Middle East's sole if undeclared nuclear-weapon state, which sees its very existence threatened by a nuclear Iran, and which has made no secret that it may resort to military action.

Prime Minister Benjamin Netanyahu, who also has an election to win in January, told the UN General Assembly in September that "at most by next summer" Iran would have enough 20-percent uranium to "move on to the final stage."