Iran’s supreme leader Ayatollah Ali Khamenei has dismissed a US offer of one-to-one talks on Tehran’s nuclear programme.
Ayatollah Ali Khamenei said in a speech posted online that the US was proposing talks while “pointing a gun at Iran”.
On Saturday, US Vice-President Joe Biden suggested direct talks, separate to the wider international discussions due to take place later this month.
But the US widened sanctions on Iran on Wednesday, aiming to tighten a squeeze on Tehran’s ability to spend oil cash.
Iran’s supreme leader Ayatollah Ali Khamenei has dismissed a US offer of one-to-one talks on Tehran’s nuclear programme
Iran and P5+1 are meeting in Istanbul, Turkey, for talks aimed at ending the deadlock over Tehran’s nuclear programme.
Envoys from the US, UK, France, China, Russia and Germany hope the talks will halt rising regional tension.
Iran says its nuclear programme is peaceful, but critics suspect it of seeking to develop nuclear weapons.
Israel has hinted in recent months that it could carry out a pre-emptive strike on Iran to prevent that happening.
The talks are the first to be held between Iran and the world’s major powers for 15 months.
The envoys are setting the bar pretty low – they do not expect detailed, substantive proposals from either side.
What they want to see is whether Iran is ready to seriously engage and, if that happens, there may be another round of talks in four to six weeks time.
Michael Mann, spokesman for EU foreign policy chief Catherine Ashton, said the Istanbul talks had started in a “positive atmosphere”, adding: “There is a desire for substantive progress.”
The six world powers, known collectively as the P5+1, hope the talks will at least cool tensions.
Iran and P5+1 are meeting in Istanbul for talks aimed at ending the deadlock over Tehran's nuclear programme
Russia said they had to be “constructive”, and warned both sides not to “overblow the differences” between them.
“We really need to find a middle course,” said Deputy Foreign Minister Sergei Ryabkov.
“The negotiations are about renewing confidence.”
US President Barack Obama has described this as a “last chance” for diplomacy to work, while Secretary of State Hillary Clinton said Iran had to “demonstrate clearly in the actions they propose that they have truly abandoned any nuclear weapons ambition”.
But one source close to the Iranian delegation told Agence France-Presse: “So far the Iranian delegation finds the Western position … disappointing and discouraging.”
On Thursday, Iranian President Mahmoud Ahmadinejad said his country was “standing firm on its fundamental rights and under the harshest pressure will not retreat an iota from its undeniable right”.
The P5+1 group hopes eventually to persuade Iran to reduce its enrichment of uranium and fully open up its nuclear facilities to inspectors from the International Atomic Energy Agency (IAEA).
There are suggestions that the stringent sanctions on Iran could be reduced if it complies with the requests.
The last series of international talks broke down in January 2011 after the parties failed to agree on any issues.
Since then, the IAEA expressed concern that Iran had failed to co-operate with its inspectors and had carried out activities “relevant to the development of a nuclear explosive device”.
Israel, which believes a nuclear-capable Iran would be a direct threat to its security, has warned that time is running out to prevent that outcome.
Prime Minister Benjamin Netanyahu has said he would never allow Israelis to “live in the shadow of annihilation”, and hinted his country is ready to strike Iran’s nuclear facilities if diplomacy does not work soon.
President Barack Obama has warned against “loose talk of war”, while stressing that all options remain open.
The European Union has agreed on new sanctions on 180 Iranian officials and firms over Tehran’s controversial nuclear programme.
EU ministers meeting in Brussels also agreed to work on other measures that could target Iran’s energy sector.
The new sanctions follow a UN report linking Iran to the development of a nuclear weapon. Iran denies the claims.
The EU measures also come two days after hundreds of Iranian protesters stormed the UK embassy in Tehran.
Britain announced on Wednesday it was expelling all Iranian diplomats from London, after pulling its own out of Tehran.
The European Union has agreed on new sanctions on 180 Iranian officials and firms over Tehran's controversial nuclear programme
A spokesman for EU foreign policy chief Catherine Ashton said that the new sanctions would target 39 people and 141 companies and would include the freezing of assets and travel bans.
EU ministers said in a statement: “The council agreed to broaden existing sanctions by examining, in close co-ordination with international partners, additional measures including measures aimed at severely affecting the Iranian financial system, in the transport sector, in the energy sector.”
Foreign ministers failed to agree on an oil embargo against Iran because some EU countries are dependent on Iranian oil.
Ministers said a decision on future measures would be taken no later than January.
EU officials say the latest sanctions are not linked to the storming of the British Embassy in Tehran on Tuesday.
However, ministers denounced the attack and said the EU would take “appropriate measures in response”.
The EU statement did not specify what those would be.
Ahead of the Brussels talks, UK Foreign Secretary William Hague said he wanted “an intensification of the economic pressure”, particularly the isolation of Iran’s financial sector.
Last week the US, Canada and the UK announced new sanctions against Iran in the wake of the report from the UN nuclear watchdog, the International Atomic Energy Agency (IAEA), that said Iran had carried out tests related to “development of a nuclear device”.
Iran insists that its nuclear programme is solely for peaceful purposes.
The 27-member EU has already frozen the assets of hundreds of Iranian companies and has adopted measures to prevent new investment and technological assistance to Iran’s gas producing and refining industry.
The EU is yet to release details about the 180 officials and entities targeted by the sanctions.
Despite the IAEA report, Iran was not referred to the UN Security Council because Russia and China were opposed to the move.
Iran expressed regret for the attack on the UK embassy – and another UK diplomatic compound in Tehran – and said a number of protesters had been arrested.
However, the semi-official Fars news agency said on Thursday that police had freed 11 people held over the attacks. There was no explanation for their release.