Iran has rejected a request by nuclear inspectors to pay a second visit to a military base, the UN's International Atomic Energy Agency (IAEA) has said.
The IAEA is urging Iran to be more open to inspections
The US suspects nuclear experiments aimed at building an atomic bomb might be under way at facilities in Parchin.
While UN inspectors were allowed to visit the site in January, they were kept away from a number of buildings.
But the agency acknowledged Tehran had allowed access to nuclear material and facilities "in a timely manner".
Even though Iran is not required to allow access to sites where there is no clear indication of ongoing nuclear experiments, Western diplomats believe that by allowing such inspections, Iran would prove it was not working on a bomb.
But in a written statement to the IAEA on Sunday, Iranian authorities said there was "no justification for any additional visit" to the site, 30km (20 miles) south-east of Tehran.
The agency's deputy director, Pierre Goldschmidt, also mentioned Iran's refusal to answer questions about another facility, the Lavizan site in the capital.
He also said that a December visit to an uranium conversion facility at Isfahan, inspectors had found an extensive tunnel was being dug under the plant.
Iran, which argued the tunnel would be used to store equipment in case of a US or Israeli attack, had failed to report its intention to go ahead with the extensive excavation work.
Finally, he said Iran had refused to back down on plans to build a heavy-water reactor which could produce plutonium for bomb-making.
Iran has always insisted it is carrying out atomic research merely to produce electricity.
But Britain, France and Germany have been trying to persuade it to stop producing enriched uranium, which can be used both for civil and military purposes, and accept a package of incentives in return.
On Sunday, the IAEA's director-general, Mohamed El Baradei, called on Iran to step up its collaboration with the inspectors, and warned that the inquiry into its nuclear potential might be lengthy.