The UN nuclear watchdog chief said on Monday there were indications of "activities" taking place at an Iranian military site which his inspectors want to visit as part of an investigation into Iran's nuclear programme.

Yukiya Amano, director general of the International Atomic Energy Agency, did not give details about what may be occurring at Parchin nor say whether it could be an attempt to conceal any evidence ahead of a possible IAEA visit.

The Vienna-based UN agency asked for access to Parchin during two rounds of talks with Iran earlier this year, but the request was not granted.

"We have some indication that activities are ongoing at the Parchin site. It makes us believe that going there sooner is better than later," Amano told a news conference.

His comments confirmed comments made by IAEA-accredited diplomats to Reuters last week, with one referring to reports "we have heard about possible sanitation" of the Parchin facility that he called "very concerning".

Last year's IAEA report laid bare a trove of intelligence pointing to research activities in Iran relevant for developing the means and technologies needed to assemble nuclear weapons, should it decide to do so. 

One key finding was information that Iran had built a large containment chamber at Parchin southeast of Tehran in which to conduct high-explosives tests that the IAEA said are "strong indicators of possible weapon development".

Amano also confirmed that Iran  had offered access to another site mentioned in an IAEA report in November that pointed to a nuclear weapons agenda in the country.

Diplomats told Reuters last week that Iran had made a last-minute offer, during talks in Tehran last month, of a visit to a site in Marivan but that this had been turned down by the IAEA.

Amano said the IAEA team, led by the agency's global chief of inspections, only learnt about the alternative site a few hours before it was due to leave Tehran. "With that we cannot do a serious job. That is why we didn't go to Marivan," he said.

Iran says its nuclear programme is entirely peaceful and rejects allegations of planned weapons as forged and baseless. - Reuters