Russia's foreign minister and the US secretary of state are headed to Geneva to participate in negotiations on Tehran's nuclear programme as envoys from the seven countries involved struggle to seal a deal.
John Kerry and Sergei Lavrov will join diplomats from Iran and six world powers, including Russia and the US, who are trying to reach an agreement that would see Tehran roll back on its atomic activities in exchange for sanctions relief.
Lavrov has already arrived in the Swiss capital as Kerry was scheduled to leave for Geneva later on Friday.
The Chinese Foreign Minister Wang Yi is also headed to join the talks in Geneva on Saturday, along with the British Foreign Secretary William Hague, German Foreign Minister Guido Westerwelle and the French Foreign Minister Laurent Fabius.
The US State Department spokeswoman Jen Psaki has warned that Kerry's move should not necessarily be taken as a sign that any deal with the Iranians was imminent.
"Even if the secretary travels, it is not a prediction of the outcome," Psaki said.
Differences on whether Iran has the right to enrich uranium that could be used to make nuclear weapons appeared to be a key sticking point on Friday between two top negotiators.
Mohammad Javad Zarif, the Iranian foreign minister, and Catherine Ashton, the European Union's top diplomat, have met repeatedly since Wednesday to agree on language in a document that could pave the way for a nuclear deal acceptable to both Tehran and the six, the five UN Security Council permanent members and Germany.
Iran, reeling from crippling sanctions imposed by the US, the EU and the UN, says it is enriching uranium only for reactor fuel, medical uses and research. But the technology can also produce nuclear warhead material.
Zarif last weekend indicated that Iran was ready to sign a deal that does not expressly state Iran's right to enrich, raising hopes that a deal could be sealed at the current Geneva round.
On Wednesday, however, Iran's supreme leader, Ayatollah Ali Khamenei, said his country would never compromise on "red lines".
Since then Tehran has reverted to its original line - that the six powers must recognise this activity as Iran's right under the Nuclear Non-proliferation Treaty to which Tehran is a signatory, despite strong opposition by Israel and within the US Congress.
A senior Iranian negotiator said that the Iranian claim did not need to be explicitly recognised in any initial deal, despite Khamenei's comment. He did suggest, however, that language on that point remained contentious, along with other differences. He demanded anonymity because he was not authorised to discuss the diplomatic manoeuvering.
The United States and its allies have signalled they are ready to ease some sanctions in return for a first-step deal that starts to put limits on Iran's nuclear programme.
But they insist that the most severe penalties - on Tehran's oil exports and banking sector - will remain until the two sides reach a comprehensive agreement to minimise Iran's nuclear arms-making capacity.
The current talks are the third since Iran's President Hassan Rouhani's election in June and are seen as the biggest hope in years to resolve the decade-old standoff over Tehran's nuclear ambitions.
Failure might mean Iran resuming the expansion of its atomic activities, Washington and others adding to already painful sanctions, and possible Israeli military action.