Iran urged to enter 'serious' nuclear dialogue
Six world powers have urged Iran to enter "serious dialogue" on its nuclear programme "without preconditions".
The US, UK, France, Germany, Russia and China called on Iran to co-operate fully with UN inspectors and allow them to visit the Parchin military site.
The countries have until now disagreed on their approach to Iran, with the West seeking a tougher line and Russia and China urging conciliation.
Iran says its nuclear programme is purely for peaceful purposes.
But it is widely thought to be seeking to develop nuclear weapons.
On Wednesday, diplomats raised concerns about the secret military development site at Parchin, amid speculation of possible clean-up activities there.
The six countries, known as the P5+1, are attending the talks at Vienna headquarters of the UN's nuclear watchdog the International Atomic Energy Agency (IAEA) to prepare for future talks with Iran.
An IAEA board meeting had been adjourned for one day on Wednesday to allow for further talks after the envoys failed to agree on what line to take towards Iran.
Earlier this week the six powers accepted an Iranian offer to return to talks for the first time in over a year. No date or venue has been set.
After Thursday's discussions, the powers released a joint statement saying: "We call on Iran to enter, without pre-conditions, into a sustained process of serious dialogue, which will produce concrete results."
The statement said negotiations would "address the international community's long-standing concerns and that there will be serious discussions on concrete confidence building measures".
American envoy to the IAEA, Robert Wood, said Iran must now respond to international demands.
"If it does not provide the agency with the necessary co-operation, and I mean concrete co-operation, then the board of governors at its next meeting in June will have to look at further steps," he said.
"Iran is now on notice, it's been put on notice that it needs to comply with its obligations and it needs to do so forthwith."
The group also expressed concern that two previous visits to Iran by IAEA inspectors had failed to resolve questions the agency had, and urged Iran to co-operate in particular over the Parchin site.
The IAEA has previously said it suspects the Parchin site may be being used for nuclear weapons related testing.
IAEA inspectors last visited Parchin in 2005 and in February this year they were turned away despite "intensive efforts" to visit.
Iran suggested on Monday that it would be prepared to grant UN weapon inspectors access to Parchin in the future but the IAEA head said on Thursday that it had had no formal contact from the country about allowing access to the site.
The complex, south of Tehran, is dedicated to the research, development and production of ammunition, rockets and explosives.
On Wednesday, IAEA diplomats told the Associated Press that satellite images of the site suggested trucks and earth-moving equipment were being used, possibly to clear up traces of nuclear tests.
Two unnamed diplomats told the AP news agency that Iran could be trying to cover up tests of a neutron device used to set off a nuclear blast.
The IAEA talks comes amid growing speculation that Israel is planning to unilaterally carry out a pre-emptive strike on Iran's nuclear facilities.
On a visit to Washington this week, Israeli Prime Minister Benjamin Netanyahu said time was running out to prevent Iran from obtaining nuclear weapons.
US President Barack Obama has criticised the "loose talk of war" - he has insisted there is still time to solve the crisis diplomatically, but has refused to rule out a military option.
On Thursday, Iran's Supreme Leader Ayatollah Ali Khamenei welcomed Mr Obama's attempts to dampen the calls for a strike on Iran, calling them "an exit from illusion".
The US and Israeli leaders did not, however, discuss an Israeli request for advanced US military technology that could be used against Iran, the White House said.
The denial came after an Israeli official suggested the country had asked the US for advanced "bunker-buster" bombs and refuelling planes that could improve its ability to attack Iran's underground nuclear sites.