Ministers from six world powers are expected to hold their first full session on Monday with Iran's foreign minister over its nuclear programme.
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.
BBC
