The UN atomic watchdog said Thursday that a visit proposed by Iran to the northwestern Marivan region would not be helpful to investigating allegations of large scale high explosive experiments.
The International Atomic Energy Agency has "explained clearly to Iran -- on more than one occasion -- that an offer of a visit of Marivan does not help address specific concerns related to the issue of large scale high explosive experiments," spokesman Serge Gas said.
The IAEA conducts regular, close inspections of Iran's nuclear programme.
But it has been pressing Iran for years to address allegations that prior to 2003, and possibly since, Tehran conducted research into developing nuclear weapons. The suspected activities in Marivan form part of that.
Iran has consistently rejected the claims, set out in a major IAEA report in 2011 based mostly on information it judges "broadly credible" provided by unnamed other countries, as baseless.
Iran says the information is based on intelligence reports from Israel and the United States, intelligence that it says it has not been able to examine itself, and some analysts are also sceptical.
Progress began to be made last year but the probe has since stalled, with Iran failing to provide information by an August 25, 2014 deadline on two out of around a dozen suspect areas.
Last month Iran's envoy to the IAEA, Reza Najafi, made the offer of a visit to Marivan, and on Thursday he told reporters that this was "to show that the claims are baseless".
"It is up to those member states who have given the information to the agency to provide the details to the agency, to enable the agency to verify their claims," he said.
Separately, Iran and the five permanent members of the UN Security Council plus Germany are seeking a landmark deal by June 30, 2015 but this concerns Iran's current and declared activities, most notably its uranium enrichment facilities.
But the US envoy to the IAEA, Laura Kennedy, said in September that the watchdog's allegations of the "possible military dimensions" to Iran's programme "must be addressed as part of any comprehensive" wider deal between Iran and world powers.