Secretary of State John Kerry has arrived in Geneva for Iran nuclear talks involving the UK, Russia, France, China and Germany after three days of lower-level meetings.
The foreign 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 John Kerry, who arrived in Geneva early on Saturday, had the goal of “continuing to help narrow the differences and move closer to an agreement”.
John 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.
The other ministers from the so-called P5+1 group of nations were also arriving on Saturday.
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.”
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