US Secretary of State John Kerry said no agreement has yet been reached on Iran's nuclear programme, as he arrived in Geneva.
He joined British, French and German foreign ministers on Friday for unscheduled talks with Iranian representatives.
Under a deal that has been floated Iran could freeze expansion of its nuclear activity for limited sanctions relief.
However, a BBC correspondent says although the parties appear closer to a breakthrough than during previous talks, the outcome still remains uncertain.
Mr Kerry interrupted the itinerary of a trip to the Middle East and North Africa to fly to Geneva after meeting Israeli Prime Minister Benjamin Netanyahu in Jerusalem.
On arrival he said: "I am delighted to be here at the invitation of (EU Commissioner) Catherine Ashton to try to work with colleagues to see if we can narrow some differences.
"There are still some things on the table that are unresolved. I want to emphasise there is not an agreement at this point.
"(We) hope to try to narrow the differences but no-one should mistake that there are important gaps to be closed."
The IRNA news agency reports Friday prayer leaders across Iran urged the public to support the nuclear negotiating team.
Since 2006 the UN Security Council has imposed a series of sanctions - including asset freezes and travel bans - on entities and people involved in Iran's nuclear programme.
Separate US and EU sanctions have targeted Iran's energy and banking sectors, crippling its economy.