(Reuters) - Facing an imminent toughening of sanctions, Iran is hinting at a readiness to give some ground in its long nuclear stand- off with world powers, but any flexibility could split their ranks and lead to protracted uncertainty about how to respond.

The stakes are high, for the longer the impasse goes on, the closer Iran will get to the technological threshold of capability to develop atomic bombs, raising the odds of last-ditch Israeli military strikes on its arch-foe and the risk of a new Middle East war a troubled global economy cannot afford.

A succession of optimistic statements by Iranian officials and academics has raised speculation that Tehran may offer concessions to its six main negotiating partners in talks scheduled for May 23 in Baghdad, a move that could ease regional tensions and soothe fears of a fresh spike in oil prices.

Such an offer would also be closely studied by Israel, which has threatened to use force to destroy nuclear installations the Islamic Republic says are purely civilian in nature but the West suspects are geared to gaining a weapons capability.

Any talk of a diplomatic breakthrough, though, is almost certainly premature.

Whatever concrete gestures are tabled by Iran would test anew the cohesiveness of joint Western, Russian and Chinese efforts to prevent an Iranian atom bomb capability, and might simply lead to months of inconclusive consultations among its interlocutors about how to answer Tehran´s move, analysts say.

Differences in how best to match an Iranian offer - for example by suspending some sanctions in return for Iran shelving enrichment of uranium to 20 percent purity, a level that worries U.N. nuclear experts - could snag efforts to turn any such initiative into meaningful movement towards negotiations.

"Don´t expect a ‘Kumbaya´ (celebratory) moment. It´s going to be a poker play" between Iran and the major powers, French analyst Bruno Tertrais said. "I would be surprised if what happens in Baghdad was more than an agreement on interim steps."

ISOLATION

There is "no doubt " that Iran´s policy would be to split the six, known as the P5+1, says Dennis Ross, until November a chief Middle East strategy adviser at the White House.

"I also have no doubt that they probably will put something on the table that they think will be attractive to some of the members of the P5+1," Ross told an audience at the Bipartisan Policy Center in Washington.

He said one such move could be Iranian assurances on a halt to stockpiling of 20 percent enriched uranium.

That level, well beyond the 5 percent of fissile purity suitable for running civilian nuclear power plants, is intended only to replenish the fuel stocks of a medical isotope reactor, Iran says. But it also moves Iran farther down the road towards the highly enriched grade of uranium usable in bombs.

One Western government assessment is that it would take Iran two to three years to manufacture a usable nuclear weapon in the event that authorities in Tehran decided to attempt that task.

Analysts and some diplomats have said Iran and the global powers must compromise for any chance of a long-term settlement, suggesting Tehran could be allowed to continue limited low-level enrichment if it accepts more intrusive U.N. inspections.

But Iran has often managed to limit its diplomatic and economic isolation by sowing rifts among the six states spearheading international efforts to rein in Iran´s nuclear program, leading to a watering-down of U.N. sanctions.

Western analysts are on alert for any new such gambit now.

A united front among Russia, China, the United States, France, Germany and Britain is the most powerful leverage the outside world has in ensuring Iranian compliance with international safeguards intended to curb the proliferation of nuclear weapons, Western analysts say.

And yet that unity has always been fragile.

Russia and China, which both have strong trade ties to Iran, have supported four rounds of U.N. sanctions imposed since 2006 on Iran over its refusal to suspend enrichment-related activity and grant unfettered U.N. inspections to resolve suspicions of military dimensions to its nuclear program.

But Moscow and Beijing criticized the United States and the European Union last year for meting out extra unilateral sanctions against Iran. Russia has made clear its opposition to any further U.N. Security Council measures against Tehran.

"I think P5+1 will have significant problems whenever it comes to Iran actually moving and how they respond," a European diplomat told Reuters. "At this moment in time it is easy and nothing has been promised by Iran ... but I think it will become very difficult and very tense on the P5+1 side once they have to start reacting to an Iranian step."

"EARLY TEST OF UNITY"

Mark Fitzpatrick of London´s International Institute for Strategic Studies said an Iranian demand for an easing of sanctions in return for its concessions "will present an early test of P5+1 unity. For the West, any lifting of sanctions would require significant limitations on the enrichment program."

There is little debate about what may be encouraging Iran to indicate new flexibility: Iran, analysts say, wishes to stave off the planned July 1 start to a European Union ban on imports of Iranian oil, a significant measure since the EU takes a fifth of the country´s petroleum shipments.

But there is plenty of speculation about the extent to which Russia and China are prepared to reward any Iranian shift.

Shashank Joshi of the Royal United Services Institute said divergence between Russia and China and its other partners would likely emerge on the price the world should demand for dropping the insistence, enshrined in the Security Council resolutions, that Iran cease any enrichment whatsoever.

He said the United States would want to see the dismantling of an enrichment plant buried deep under a mountain at Fordow south of Tehran, the Iranian nuclear site best sheltered from any possible air strike.

"The Russians and Chinese may recognize that this is unlikely, and may accept Iranian offers short of this," he said.

"So we should expect to see Iran attempt to split the Russians and Chinese from the others by offering something concrete and significant, but short of dismantlement."

Tehran has ruled out closing the bunkered Fordow site.

SIGNS OF NEW IRANIAN APPROACH

Diplomats and analysts say an agreement is still far off, but the signs are growing that Iran´s leaders are changing their approach and preparing public opinion for a potential shift.

Tehran´s former chief nuclear negotiator, Hossein Mousavian, now a visiting scholar at Princeton University in the United States, said last month Iran and major nations had a "historic opportunity" to settle their decade-old nuclear dispute.

On May 2, Deputy Foreign Minister Mohammad Mahdi Akhondzadehhe said in a speech in Vienna: "We continue to be optimistic about upcoming negotiations."

In April, Foreign Minister Ali Akbar Salehi said Iran was "ready to resolve all issues very quickly and simply".(Editing by Mark Heinrich) (© Thomson Reuters 2012. 05/16/12)