The deadline for reaching an agreement
is Tuesday.
The essence of an emerging deal is to
keep Iran at least one year away from being able to produce enough nuclear fuel
for a single weapon.
Representatives from the US, Germany,
France, the UK, China and Russia are at the talks in Lausanne, Switzerland.
The
two sides have explored compromises in areas including the number of centrifuges
used to enrich uranium that Iran could operate, and its nuclear enrichment work
for medical research.
Restrictions on Iran's nuclear work
would last at least a decade, and include rigorous inspections, in exchange for
an end to crippling sanctions, says the BBC's Barbara Plett Usher in Lausanne.
Despite progress, disagreements remain
on how long Iran would be subject to constraints, and on the speed of
sanctions' relief, our correspondent says.
At their meeting with Iranian Foreign
Minister Mohammad Javad Zarif, the other representatives will be aiming to
agree on a political framework by Tuesday night that would set the stage for
technical talks.
US officials said all parties have
agreed to a "step by step approach" to reaching a deal, but sticking
points remain.
