The foreign ministers of six major powers are meeting -again - in Geneva to negotiate a nuclear deal with Iran.
US Secretary of State John Kerry has arrived for talks involving the UK, Russia, France, China and Germany after three days of lower-level meetings.
The ministers hope to close a deal for Iran to curb uranium enrichment in return for a loosening of sanctions.
But Iran insists it must be allowed to enrich uranium for power stations, and denies it is seeking nuclear weapons.
Some US politicians say they will push for more sanctions if the talks fail.
Negotiators have been working since Wednesday to try to find an agreement that is acceptable to both sides.
The talks had been scheduled to conclude on Friday, but were extended amid hopes of a possible breakthrough.
The state department said Mr Kerry, who arrived in Geneva early on Saturday, had the goal of "continuing to help narrow the differences and move closer to an agreement".
Mr Kerry's participation in itself does not prove a deal is at hand, but it does show that the talks may have reached a critical stage, says the BBC's James Reynolds in Geneva.
The other ministers from the so-called P5+1 group of nations were also arriving on Saturday.
Hard line from French
French Foreign Minister Laurent Fabius told reporters as he arrived: "I want a deal, but a solid deal, and I am here to work toward that end."
France has taken a harder line on Iran than other Western powers.
EU foreign policy chief Baroness Catherine Ashton is leading the conference.
On Friday she briefly met Iranian Foreign Minister Mohammed Javad Zarif for a conversation that Iran's official Irna news agency described as "complicated and tough".
Analysts say a major sticking point is Iran's insistence on its right to enrich uranium - a process that yields material used to manufacture fuel for power stations, but can also be used in weapons.
Western diplomats are also concerned about a reactor Iran is building at Arak - an issue which disrupted the first round of talks.
US President Barack Obama has said any interim agreement would see the bulk of international and US sanctions remain, but that Iran would get sanctions relief worth between $6bn and $7bn.
The essence of the deal would involve Iran making no more advances in its nuclear programme and agreeing to "more vigorous inspections", he said.