UN nuclear watchdog chief Mohamed ElBaradei has failed to convince Iran to freeze its nuclear programme during a brief visit to Tehran.
But he said both sides had agreed to continue an intensive dialogue over the next few weeks on the issue.
Iran announced two days ago it had succeeded in enriching uranium and has vowed not to back down.
The US said that when the UN Security Council reconvened there would have to be "consequence... for that defiance".
'Reasonable and logical'
Mr ElBaradei said his inspectors had taken samples to check to what degree Iran has successfully enriched uranium.
The results of the samples will be reported back to the board of the International Atomic Energy Agency (IAEA).
After meeting Iran's nuclear negotiator, Ali Larijani, Mr ElBaradei stressed that there was still time to negotiate a settlement by which "Iran's needs for nuclear power is assured and the concern of the international community is also...put to rest."
The IAEA head said they had a good discussion about "confidence-building measures", including a call by the UN Security Council for Iran to suspend all its nuclear activities.
Western nations suspect Iran of wanting to develop a nuclear weapon, but Tehran insists its plans are for a peaceful, civilian energy programme only.
Mr ElBaradei is to report back to the UN Security Council at the end of this month on whether Tehran is complying with its demand to stop all enrichment activity by 28 April, or risk isolation.
So far, Iran has adamantly refused to roll back its nuclear programme, the BBC's Francis Harrison in Tehran says.
Iran's position is that it is happy to co-operate with international inspections of its nuclear sites but will not stop its drive to produce nuclear fuel on an industrial scale, our correspondent adds.
Debate in Iran
Mr Larijani indicated, after his meeting with Mr ElBaradei, that the UN's demand for a return to a freeze of its nuclear programme was not the way to solve the problem.
"Every action must be reasonable and logical. We are cooperating in a constructive manner" with the IAEA, "so such a proposal is not very important to solve the problem," he said.
NUCLEAR FUEL CYCLE
Speaking as Mr ElBaradei arrived in Tehran, Iranian President Mahmoud Ahmadinejad said: "Our answer to those who are angry about Iran obtaining the full nuclear cycle is one phrase, we say: Be angry and die of this anger."
"We will not hold talks with anyone about the Iranian nation's right [to enrichment] and no one has the right to step back, even one iota," he said.
The US and Europe are pressing for sanctions against Iran, a step UN Security Council members Russia and China have opposed.
On Thursday, US Secretary of State Condoleezza Rice said the council would have to look at measures to ensure Iran complied with its international obligations.
"When the Security Council reconvenes there will have to be some consequence for that action and that defiance. We will look at a whole range of options available to the Security Council," Ms Rice said.
A senior Chinese arms control official, Assistant Foreign Minister, Cui Tiankai, is due in Tehran for talks on Friday.
The BBC's Daniel Griffiths in Beijing says China has so far kept a low profile but it is increasingly keen to be seen as a responsible, international player, and Iran is a perfect opportunity to strengthen those credentials.
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