Iran said Thursday it held "constructive" talks with a visiting delegation of the UN nuclear watchdog seeking to resolve outstanding issues in Tehran's disputed atomic programme.

Tero Varjoranta, deputy director general of the International Atomic Energy Agency, led the team which held talks Tuesday and Wednesday on two final points on which the IAEA is still seeking explanations from Iran.

The two questions focus on concerns that the Islamic republic's nuclear activities had military dimensions.

"During these two days, all the bilateral issues were discussed, in particular, how to carry out the agreed measures and the ways forward were discussed," said Iran's ambassador to the IAEA, Reza Najafi.

"These negotiations were constructive in terms of content. They were also direct," he said, quoted on the state television's website.

The Vienna-based IAEA said that the "technical meetings" did not resolve the two outstanding issues.

"The Agency and Iran will continue discussions on these measures" at a date to be confirmed, the body added in a statement on Thursday.

The IAEA disclosed in September that Iran had failed to meet an August 25 deadline to provide information on five points meant to allay fears it was developing nuclear weapons.

Iran insists its nuclear programme is entirely peaceful.

One of the IAEA's questions centres on Iran's purported experiments with large-scale high explosives.

Under an agreement reached in November 2013 with the IAEA, Iran has already responded to 16 of the 18 issues the agency identified as relevant to its nuclear activities.

Satisfying the IAEA's concerns is considered crucial to a hoped for conclusion by November 24 of a comprehensive nuclear agreement with the United States and other world powers.

In a letter to the IAEA, Najafi said the delay was due to the "complexity" of the issues and that allegations of nuclear-orientated experiments lacked credibility and any solid proof.

The visit by the watchdog's delegation came after a deadly fire on Sunday at a defence ministry plant used to produce explosives in an area east of Tehran that also houses several other military sites.

A defence ministry official on Wednesday denied reports published in foreign media of an explosion at the Parchin base, which UN nuclear inspectors have been seeking to visit to answer concerns about Iran's atomic programme.

The base lies at the centre of allegations of past Iranian research into sophisticated explosives that can be used to detonate a nuclear warhead.

The Institute for Science and International Security, a US research centre, said the blast could have hit a site at the south of the Parchin complex.

"Two buildings that were present in August 2014 are no longer there, while a third building appears to be severely damaged. In total at least six buildings appear damaged or destroyed," the centre said after analysing satellite imagery.