UN Secretary General Kofi Annan is to visit Iran next Saturday, the Iranian foreign ministry has confirmed.
Iran's president opens a new phase in a heavy water project
The visit will come two days after a UN deadline for Iran to suspend uranium enrichment and amid fears it is trying to develop a nuclear bomb.
On Sunday Iran repeated it would go ahead with uranium enrichment - but only to fuel a nuclear power programme.
Iran's foreign minister again called for European powers to "return to talks without prejudgement".
News of the visit comes amid fresh tension over Iran's nucelar ambitions. On Saturday Iranian President Mahmoud Ahmadinejad inaugurated a facility producing coolant for nuclear reactors.
And last Tuesday, Iran responded at length to a package of incentives offered in exchange for suspension of its uranium enrichment programme.
The details of Iran's response to the offer were not made public, though the US said it "fell short" of the full and verifiable suspension of uranium enrichment demanded by the UN by 31 August - and that the "next steps" would have now to be considered.
Those "next steps" will include the threat of sanctions, say correspondents.
China and Russia have stressed their preference for a negotiated solution to the crisis.
Iran's foreign ministry spokesman Hamid Reza Asefi confirmed that Mr Annan would be visiting Tehran on 2 September.
Mr Annan cancelled his last scheduled visit to Tehran in November, saying it would be "inappropriate" in the wake of comments by Mr Ahmadinejad which appeared to call for Israel to be "wiped off the map".
Some specialists have since questioned the accuracy of that translation.
Mr Asefi also said it was "about time for the European side to return to talks without any prejudgement. Serious talks can lead us to reach an understanding," he said.
However, the country's chief nuclear negotiator Ali Larijani again underlined Iran's commitment to continuing to develop its nuclear programme.
"Production of nuclear fuel is one of Iran's strategic objectives," Mr Larijani told state radio.
"Any action to limit or deprive Iran could not force Iran to give up this goal."
State television also reported that Iran had successfully test-fired a long-range, radar-evading Sagheb missile in Gulf waters on Sunday.
'Not a threat'
President Mahmoud Ahmadinejad said Iran would never abandon its nuclear programme, but insisted nuclear weapons were not its goal.
"We are not a threat to anybody, even the Zionist regime which is a definite enemy of the people of the region," he said.
Heavy water made at Arak, 190km (120 miles) south-west of Tehran, will be used to cool a reactor being built that will create a plutonium by-product that could be used to make atomic warheads.
Iran points out that as a signatory to the nuclear Non-Proliferation Treaty (NPT) it is entitled to a nuclear power programme and says it has not broken any rules.
But the Western powers accuse Iran of concealing an enrichment programme, and Washington has refused to rule out military action.