Talks on inspections without notice of Iran's nuclear facilities have begun in Tehran.
The recently inaugurated Shahab-3 missile added to international fears
A team of legal experts from the UN's nuclear watchdog is seeking to persuade it to drop opposition to such visits.
Iran said in the past it would only agree to them if it was granted more access to nuclear technology.
Washington believes Iran is secretly developing nuclear weapons under the cover of a civilian programme based on Russian technology.
Lawyers from the International Atomic Energy Agency (IAEA) sat down to talks with counterparts from Iran's Atomic Energy Organisation.
"For a country like Iran that has declared a large nuclear programme, it's very important that the inspectors are on the ground to be able to verify fully the nature of that programme," said the IAEA's Lothar Weedekind.
Iran said the final decision on accepting snap visits - the so-called additional protocol of the Nuclear Non-Proliferation Treaty (NPT) - would rest with the Islamic Republic's spiritual leader, Ayatollah Ali Khamenei.
The IAEA team was set to explain to the Iranian authorities exactly how the additional protocol works.
It "gives us greater authority in inspection and also access to information and the use of what are called verification technologies", said Mr Weedekind.
IAEA chief ElBaradei met Iranian President Khatami in July
The talks follow a visit in July by the IAEA's Director General, Mohammed ElBaradei, who had talks with Iranian President Mohammad Khatami.
Tehran asked for the new visit after coming under increasing pressure from the European Union, Russia and the US to satisfy concerns about its nuclear ambitions.
Last month, EU foreign ministers warned that relations with Iran would be reviewed if Tehran refused to co-operate with the IAEA and sign up to closer inspections.
Iran insists its nuclear programme is to generate electricity and is for peaceful purposes only.
Iranian politicians are divided over whether to allow more intrusive inspections and are facing a dilemma over how to respond to international pressure, says the BBC's Miranda Eeles in Tehran.
Iran's representative at the IAEA last month urged his country's leaders to sign up to the additional protocol.
Some politicians, however, have called on Tehran not to give in, arguing it would be better to pull out of the NPT altogether.
"The decision about joining the additional protocol will be taken based on national interests," said Iranian Government spokesman
The issue, he said, would be sent to the government for discussion, then decided by the Supreme National Security Council with final approval resting with Ayatollah Khamenei.
A second UN team is also expected this month for the last round of inspections before the release of an IAEA report on Iran's nuclear facilities in September.