Iran and UN nuclear monitors took a "major step" toward resolving remaining issues regarding the Islamic republic's disputed atomic programme, an Iranian spokesman said Tuesday.

The 24-hour visit to Iran by experts from the International Atomic Energy Agency (IAEA) on Monday followed a similar trip last week by its chief executive Yukiya Amano, but no clear outcome was reached.

Quoted by the official IRNA news agency, the spokesman for Iran's Atomic Energy Organisation, Behrouz Kamalvandi, said progress was made but he gave no details on the latest discussions.

"Iran and the IAEA took a major step in resolving the outstanding issues to reaching a fundamental understanding on the topics and the timing of cooperation," he said.

Kamalvandi described Monday's meetings as "constructive and forward-looking" and said the second IAEA visit "shows the serious determination of both sides to enhance cooperation."

However, the statement came as the latest deadline for a final nuclear deal between Iran and the P5+1 powers (Britain, China, France, Russia, the United States and Germany) seemed likely to be missed at talks in Vienna.

The IAEA suspects Iran carried out research at least until 2003 on developing nuclear weapons.

It is seeking access to scientists who might have been involved, as well as documents and the sites at which any such activities took place.

Iran denies the allegations and its supreme leader Ayatollah Ali Khamenei in a speech on June 23 ruled out granting access to military sites or nuclear scientists.