Iran agreed to allow UN access to a new nuclear plant at talks with international powers which included its highest level encounter with the United States in three decades, officials said.
Representatives of the Western powers, which have threatened new sanctions over what they suspect is a bomb-making programme, expressed cautious optimism over the Geneva negotiations. The two sides agreed to meet again this month, officials said.
But in Washington US President Barack Obama called on Iran to take "constructive" steps after the talks and warned that Washington was ready to up pressure if Tehran used delay tactics.
Iran's Foreign Minister Manouchehr Mottaki also described the talks as "constructive" but again defended his country's "absolute" right to uranium enrichment.
Iran's senior nuclear negotiator Saeed Jalili met with top envoys from the United States, Russia, Britain, China, France and Germany, known as the P5-plus-1 partners in a villa overlooking Lake Geneva.
Jalili and US envoy William Burns also held their countries' highest level talks in 30 years. They discussed the increasingly tense nuclear standoff and human rights, US officials said.
Iran, which denies it is seeking a nuclear bomb, said it would let UN nuclear watchdog inspectors visit the new enrichment plant at the holy city of Qom, probably within two weeks, said EU foreign policy chief Javier Solana.
The IAEA announced later that its outgoing chief Mohamed ElBaradei had been invited to visit Iran soon.
Access to the site had been one of the key demands from the world powers since its existence was revealed last week, fuelling suspicions that Iran is hiding a nuclear weapons programme. Recent developments in Iran
"Iran has told us that it plans to cooperate fully and immediately with the International Atomic Energy Agency on the new enrichment facility near Qom," Solana told a press conference.
"We agreed to intensify dialogue in the coming weeks," he said, adding that "this is only a start" and that "we shall need to see progress on some of the practical steps we discussed today."
US and EU officials said Iran agreed to meet again before month's end. But the United States warned Iran against using the dialogue to delay its international obligations.
"Today's meeting was a constructive beginning but it must be followed with constructive action by the Iranian government," Obama said in a short televised statement at the White House.
"We're not going to talk for the sake of talking. If Iran does not live up to its obligations, then the United States will not continue to negotiate indefinitely and we are prepared to move toward increased pressure."
Western powers have warned Iran it could face further sanctions and isolation if it fails to meet international demands.
"We expect to see swift action," said Obama. "We're committed to serious and meaningful engagement, but we're not interested in talking for the sake of talking."
US Secretary of State Hillary Clinton said it had been a "productive day, but the proof of that has not yet come to fruition."
Solana said the world powers and Iran had agreed in principle on Iranian uranium being sent to a third country to be enriched and used for a research reactor in Tehran.
The talks came only a week after the new Iranian uranium enrichment plant was disclosed and just days after Iranian missile tests again rattled international nerves.
Jalili told reporters the talks had been "good" and that Iran would arrange for the IAEA to "supervise" the new enrichment plant.
The UN Security Council has already imposed three sets of sanctions on Iran for refusing to end its uranium enrichment.
While Western powers have indicated their readiness to impose further sanctions on Iran if necessary, Russia and China have been reluctant to support further measures, although Moscow has indicated it might reconsider.
The US envoy Burns met separately with Jalili highlighting the new engagement policy favoured by Obama.
Burns used the encounter to "reiterate the international community's concerns about Iran's nuclear programme," US State Department deputy spokesman Robert Wood said in a statement.
He also told Jalili that Iran needed to take concrete and practical steps to reassure the international community that its nuclear programme is peaceful, Wood said.
It was the first direct US-Iran talks on a bilateral issue since the two sides broke relations 30 years ago, although meetings took place during president George W. Bush's term on Iraq and Afghanistan, a US official said.