UNLIKELY? TRY NATURAL:

Mr Rouhani was elected in an upset landslide victory over conservative rivals in June with promises to secure relief from international sanctions imposed because of Iran's nuclear programme.

He does not want the Syrian crisis scuttling his efforts, so can be expected to press President Bashar Al Assad to honour the chemical disarmament deal.

Even before the poison gas attack in Damascus last month that triggered the disarmament plan, there were signs of division within the Iranian regime about the strategic value of continuing to support Mr Al Assad.

Mohammad Javad Zarif, Iran's foreign minister, castigated Damascus earlier this month. "We believe that the government in Syria has made grave mistakes that have, unfortunately, paved the way for the situation in the country to be abused," he said.

And on Monday, Mr Rouhani said Iran would accept anyone elected by the Syrian people as ruler, even if that is not Mr Al Assad.

Tehran also has a genuine abhorrence of chemical weapons and has vociferously condemned their use in Syria. In the 1980s, Saddam Hussein's Iraq subjected Iran to the worst chemical weapon attacks since the First World War, killing thousands of soldiers and civilians while the West turned a blind eye.

Mr Obama said last week that the Russian plan "may have a chance of success" because "Syria's allies, like Iran, detest chemical weapons" and it may be that Mr Al Assad is "under pressure from them as well".

Although on opposite sides of the Syrian conflict, Iran shares an interest with the US and its Arab rivals in ensuring that militants affiliated to Al Qaeda, who are spearheading the battle against the Assad regime, do not come to power.

For these reasons and more, many Iran experts argue that Tehran should have a seat at the long-mooted Geneva 2 talks to resolve the Syrian crisis. Saudi Arabia and hawkish US politicians oppose Iranian participation, insisting that Iran, as part of the problem, cannot be part of the solution.

But, said Hooman Majd, an Iranian-American author based in New York who knows many of the key players in Washington and Tehran, "if you want to solve a problem, then by definition you have to talk to the people you think are part of the problem". Iran "was probably instrumental in getting Assad to agree to the Russian deal".

Mr Obama on Sunday raised the prospect of Iran getting involved in the broader talks on Syria. That would be a wise move, argued a western diplomat who has served in Tehran. "Iran has resources, people on the ground and influence in Syria," the envoy said. "You can't ignore them."

Moreover, he added: "You can't ignore the parallels from 2001 when Iran was instrumental in forging the compromises that led to the formation of a new government in Afghanistan after the US-led invasion ousted the Taliban regime following the September 11 attacks."

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