US Secretary of State John Kerry will Sunday seek a breakthrough in nuclear talks with Iran, with domestic pressures in both countries now weighing heavily on hopes for a deal.

Kerry will meet Iran's Foreign Minister Mohammad Javad Zarif in Muscat, Oman, seeking to close substantial gaps that in recent months have blocked efforts to turn an interim agreement into a comprehensive settlement.

The meeting follows the revelation that US President Barack Obama reportedly wrote to Iran's supreme leader Ayatollah Ali Khamenei recently to push for a nuclear deal, arguing that 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" between the nuclear talks and other issues.

With a November 24 deadline for a final agreement just over two weeks away, Iran and the P5+1 group of world powers (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 an atomic bomb forever beyond reach.

At issue is the number of uranium-enriching centrifuges Iran should be allowed to keep spinning in exchange for sanctions relief and a transparent inspections regime at its nuclear sites. Iran wants "industrial grade enrichment" beyond its current capabilities while the West wants a reduction.

- Domestic politics overshadow talks -

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 secret talks with the US in Oman in 2012 had set the wheels in motion.

The surprise election last June of President Hassan Rouhani, who had pledged to revive Iran's sanctions-smashed economy, was a turning point 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, there will be a meeting of political directors from the P5+1 powers, also 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.