World powers and Iran have resumed talks aimed at reaching a landmark nuclear deal as Tehran's supreme leader vowed not to retreat "one step" from Iran's rights.
The opening plenary session in Geneva late on Wednesday lasted less than 10 minutes and was described by diplomats as an "introductory" meeting before delegates headed into bilateral talks.
More detailed negotiations are expected on Thursday.
Despite cautious optimism that a deal could be within reach, Secretary of State John Kerry vowed the US would not back an agreement that simply let Iran "buy time".
"We will not allow this agreement, should it be reached ... to buy time or to allow for the acceptance of an agreement that does not properly address our core, fundamental concerns," he said in Washington.
Comments earlier on Wednesday by Iran's supreme leader indicated that this week's round of talks - the third since Hassan Rouhani's election as president in June raised hopes of a deal - may be fraught.
In an anti-Israel diatribe that France said would "complicate" the Geneva negotiations, Ayatollah Ali Khamenei said: "I insist on not retreating one step from the rights of the Iranian nation."
The US, Russia, China, Britain, France and Germany, known as the P5+1, want Iran to suspend certain parts of its nuclear program for a period of several months in a "first phase", in return for easing some sanctions.
A longer-term agreement would then be hammered out that would reduce Iran's program to a size acceptable to the international community.
Iran's lead negotiator Abbas Araqchi said that "the lost confidence must be regained", in reference to the last Geneva talks 10 days ago when French objections apparently put off a long-hoped for breakthrough.
But Israel, widely assumed to have a formidable nuclear arsenal itself, has expressed alarm at the mooted deal, as have hardliners in Washington arguing for more sanctions.
Instead of stopping all uranium enrichment, the powers appear to be satisfied with the suspension of enrichment to medium levels and for stockpiles of material enriched to this level to be dealt with, as well as more IAEA inspections and a halt to work at the key Arak reactor.
For Israeli Prime Minister Benjamin Netanyahu, who arrived in Moscow on Wednesday where he sought to harden Russian President Vladimir Putin's stance, this leaves intact Iran's ability to make a bomb.
Putin said after the talks that he hoped for a "mutually acceptable" solution while the Israeli leader called for a "real solution" to the crisis.
Israel has refused to rule out bombing Iran, as it was assumed to have done with an Iraqi reactor in 1981 and a Syrian site in 2007.
Washington has sought to ease Israel's fears, insisting any deal would ensure Iran does not build atomic weapons.
"The United States and Israel share a common objective, and that is to make sure that Iran does not acquire a nuclear weapon. How we get there, we may have some tactical differences, but our objective is identical," a senior US administration official told reporters in Geneva.
Iran says its nuclear program is entirely peaceful. Uranium enrichment is the main worry for the international community since enriched uranium has civilian uses, but can also be used to build a bomb.
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