US Secretary of State John Kerry arrived Sunday in Oman to seek a breakthrough in nuclear talks with Iran, with domestic pressures weighing heavily on hopes for a deal.
Kerry will meet Iran's Foreign Minister Mohammad Javad Zarif during the day to try to close substantial gaps that have blocked the latest efforts to turn an interim agreement into a comprehensive long-term settlement.
The meeting follows the revelation that US President Barack Obama reportedly wrote to Iran's supreme leader Ayatollah Ali Khamenei to push for a nuclear deal, arguing the Islamic republic and the West have shared regional interests.
The apparent reference to the fight against Islamic State group militants in Syria and Iraq, however, was played down by Kerry in Beijing on Saturday, with the US diplomat saying "there is no linkage whatsoever" with the nuclear talks.
But with a November 24 nuclear deadline looming, Iran and the P5+1 group -- Britain, China, France, Russia and the United States plus Germany -- are far apart on what capabilities Iran's nuclear programme should have.
The West has as yet been unconvinced by Iran's denials that it has never sought a nuclear weapon -- Tehran insists its atomic activities are for peaceful, civilian energy purposes only.
A deal, for the West, aims to put a bomb forever beyond Iran's reach.
At issue is the number of uranium-enriching centrifuges Iran should be allowed to keep spinning in exchange for sanctions relief and rigorous inspections at its nuclear sites.
Iran wants "industrial grade enrichment" beyond its current capabilities while the world powers want a reduction.
However, domestic politics now hang heavily over the talks, given the loss in midterm elections of the Senate by Obama's Democrats to the Republican party, members of whom have consistently bridled at the White House's negotiations with Iran.
If talks go sour in the coming weeks it is thought the US Congress may respond with fresh sanctions on Iran. Even though Obama has the power to veto them, the prospect of new penalties could disrupt an already protracted process.
Zarif's foreign ministry is also under pressure, with members of parliament criticising the talks and threatening to scupper a deal if lawmakers themselves do not have a say in ratifying it.
Although officially supportive, hardliners in Tehran have often been ambivalent about the negotiations with the West which officially resumed last autumn after earlier secret talks with the US in Oman had set the wheels in motion.
On the plane to Muscat on Saturday, Zarif told reporters that Iran and the P5+1 have concentrated on "solutions rather than differences" since talks on the sidelines of the UN General Assembly in New York in September.
"There is still a gap between the two parties on the size of the enrichment programme and the mechanism for lifting sanctions," the official IRNA news agency quoted him as saying.
"If the other party acts with good political will, we can reach an agreement."
The surprise election last June of President Hassan Rouhani, who had pledged to revive Iran's sanctions-smashed economy, was a turning point on the nuclear issue but progress has been elusive since an interim deal came into effect in January.
After Sunday's meeting between Kerry and Zarif, which will be chaired by former EU foreign policy chief Catherine Ashton, political directors from the P5+1 powers will hold talks in Muscat, on Tuesday.
The negotiations will move back to Vienna on November 18 for a final push towards the deadline six days later.
A comprehensive agreement would represent a hard-earned foreign policy win for Obama in a region where the threat of a nuclear-armed Iran has hung for more than a decade.