US Secretary of State John Kerry has arrived in Geneva for talks on Iran's contested nuclear programme as Tehran and six world powers appeared to be on the verge of a breakthrough in the decade-old dispute.
Intense negotiations are hoping to bring a deal under which Iran would curb its atomic activity in exchange for some relief from economic sanctions.
The Russian, Chinese, French, British and German foreign ministers were due to take part along with Mr Kerry in the talks.
Diplomats said a major sticking point in the negotiations which began on Wednesday may have been overcome.
However, British Foreign Minister William Hague also cautioned that there was much work to do to bridge remaining differences.
"We [foreign ministers] are not here because things are necessarily finished," Mr Hague told reporters.
"There is a huge amount of agreement ... [But] the remaining gaps are important and we will be turning our attention to those over coming hours. They remain very difficult negotiations."
French and German foreign ministers also urged caution.
Russian foreign minister Sergei Lavrov arrived yesterday, meeting his Iranian counterpart Mohammad Javad Zarif and EU foreign policy and security chief Caroline Ashton.
Mr Kerry left for Geneva "with the goal of continuing to help narrow the differences and move closer to an agreement," State Department spokeswoman Jen Psaki said.
The decision to travel was taken after consulting with Ms Ashton, who is co-ordinating with the six nations negotiating with Iran.
Mr Kerry's appearance in Geneva is being read as a sign that a deal may be close,
Echoing US optimism, China's state-run Xinhua news agency quoted a foreign ministry spokesman as saying the talks "have reached the final moment".
The country's foreign minister left Beijing for Geneva earlier today.
France has consistently taken a tough line over Iran's nuclear programme, helping Paris forge closer ties with Irans' enemeies in Israel and the Gulf.
German Foreign Minister Guido Westerwelle said: "It's not a done deal. There's a realistic chance, but there's a lot of work to be done."
Diplomats said a compromise over Iran's insistence that its "right" to enrich uranium be internationally recognised has been proposed, possibly opening the way to a long-sought breakthrough.
The US and other western powers say there is no such thing as a right to enrich - a process that can yield both electricity and nuclear bombs - but Iran views it as a matter of national sovereignty and crucial to any deal that would resolve the stand-off over its nuclear intentions.
The Islamic Republic also wants relief from economic sanctions in return for any nuclear concessions it makes that could allay western suspicions that its nuclear fuel-making programme has military rather than its stated civilian goals.
Foreign ministers from the six nations negotiating with Iran in the previous talks three two weeks ago came close to winning concessions from Iran, which they count on to reduce the risk of Iran achieving a nuclear weapons capability.