Iran has refused to cooperate with a UN probe into potential military aspects of its atomic activities.
The International Atomic Energy Agency (IAEA) has voiced concerns about allegations that Iran carried out military-related atomic work, which the country denies.
IAEA director general Yukiya Amano wrote to the head of Iran's Atomic Energy Organisation Fereydoun Abbasi-Davani requesting help with answering the UN's queries and access to locations, equipment, documents and officials.
The IAEA, tasked with ensuring that nuclear technology is not diverted for military aims, says Iran has not engaged with the agency in substance on these issues since mid-2008.
It has investigated Western intelligence reports indicating Iran had coordinated efforts to process uranium, test explosives at high altitude and revamp a ballistic missile cone so it can take a nuclear warhead.
Abbasi-Davani reiterated Iran's often-stated position that the allegations were "fabricated and acts of forgery".
No indication was given that Iran would be prepared to heed Amano's demands, referring instead to a "work plan" agreed between Iran and the agency in 2007, which it said envisaged no inspections, meetings, or interviews with officials.
U.N. sanctions resolutions against Iran were "illegal and unacceptable" and had paved the way for the assassination of Iranian scientists and scholars, the Iranian letter said.
Two daylight bomb attacks on the same day in Tehran last November killed one nuclear scientist and wounded another. No one has claimed responsibility but Iran called it a "terrorist" attack by Israeli, British and U.S. intelligence.
The content of Iran's response to Amano highlighted the apparent deadlock in international efforts to resolve the long-running dispute over the country's nuclear programme.
"They haven't offered anything," one senior Western diplomat said about the letter from Tehran.
Another envoy said the Iran "challenge grows and grows".
Iran, already under four rounds of U.N. sanctions over its nuclear programme, denies it is seeking the bomb and says its nuclear activities aim solely at power generation, medicine and agriculture.
It has repeatedly refused to halt uranium enrichment, which can have civilian or military uses.
Amano has taken a blunter approach than his predecessor, Mohamed ElBaradei, saying in his first report on Iran in 2010 he feared it may be working now to develop a nuclear-armed missile, not just in the past.
The Japanese agency chief repeated that view in a quarterly Iran report issued last week, saying he had new information about possible undisclosed nuclear-related activities.
Diplomats said Amano's May 6 letter was a signal to Tehran to cooperate or face a possible assessment by the IAEA on the likelihood it had carried out military-linked nuclear activity, based on the evidence available to the U.N. body.
An independent assessment on whether Iran had carried out nuclear weapons research could lend weight to any renewed Western push to tighten sanctions on the major oil producer.
The latest IAEA report spelled out in more detail seven areas of particular concern where the agency wants clarification from Iran.