European Union foreign policy chief Josep Borrell was due to meet Iran's top diplomat on Saturday after arriving for talks on efforts to revive the 2015 Iran nuclear deal.
Borrell flew in to Tehran on Friday night for a surprise visit aimed at getting the talks back on track three months after they stalled amid differences between Iran and the United States.
"Diplomacy is the only way to go back to full implementation of the deal and to reverse current tensions," Borrell tweeted on Friday.
He is expected to meet Foreign Minister Hossein Amir-Abdollahian and other Iranian officials later on Saturday.
The nuclear deal has been hanging by a thread since 2018, when then US president Donald Trump unilaterally withdrew from the accord and began imposing crippling economic sanctions on America's arch enemy.
The administration of incumbent US President Joe Biden has sought to return to the agreement, saying it would be the best path with the Islamic republic.
US negotiator Robert Malley "reiterated firm US commitment to come back to the deal" on Thursday over a meal in Brussels with Borrell, according to the EU's coordinator for the talks, Enrique Mora.
The talks, which began in April last year, aim to return the United States to back the deal, including through lifting sanctions, while ensuring Tehran's full compliance with its nuclear commitments.
The negotiations stalled in March amid differences between Tehran and Washington, notably over a demand by Iran to remove its Islamic Revolutionary Guard Corps from a US terror list.
The 2015 nuclear deal reached with six major powers -- Britain, China, France, Germany, Russia and the US -- gave Iran relief from sanctions in return for guarantees it could not develop an atomic weapon.
Iran has always denied wanting a nuclear arsenal.
"We are ready to conclude this agreement, and urge Iran to seize this diplomatic opportunity to conclude now, while this is still possible," a French foreign ministry spokeswoman said on Friday.