Iranian cleric Ayatollah Ahmad Khatami warned the chief of the International Atomic Energy Agency on Monday not to become "an instrument without will in the hands of the United States" against Iran.
The hardline cleric hit out at IAEA director general Yukiya Amano in an address during communal prayers in Tehran marking the Muslim Eid al-Adha feast.
"If Mr Amano acts like an instrument without will in the hands of the United States and publishes lies by presenting them as documents, the IAEA will lose the little credibility it has left," said Khatami, an influential cleric who often leads Friday prayers in Tehran.
His comments come as the UN nuclear watchdog prepares to circulate an intelligence update among its members on Tuesday or Wednesday which is expected to focus on Iran's alleged efforts towards putting radioactive material in a warhead and developing missiles.
"The report is not going to include some sort of 'smoking gun'," one Western diplomat told AFP. "But it will be an extensive body of evidence that will be very hard for Iran to refute as forgery, as they have done in the past."
Iranian officials have already seen the Vienna-based IAEA's information, diplomats told AFP, and Foreign Minister Ali Akbar Salehi said in comments published in Iran on Sunday that it was based on "counterfeit" claims.
Amano said in a September report he was "increasingly concerned" about the "possible military dimension" of Iran's atomic activities, including those "related to the development of a nuclear payload for a missile."
Western officials cited by The Washington Post said the intelligence reinforced concerns that Iran continued to conduct weapons-related research after 2003 when, according to US intelligence agencies, Iranian leaders halted such experiments in response to international and domestic pressures.
The newspaper reported Sunday that the Iranian government has mastered the critical steps needed to build a nuclear weapon after receiving assistance from foreign scientists.
Citing unnamed Western diplomats and nuclear experts, the newspaper said a former Soviet weapons scientist had allegedly tutored Iranians on building high-precision detonators of the kind used to trigger a nuclear chain reaction.
Crucial technology linked to experts in Pakistan and North Korea also helped propel Iran to the threshold of nuclear capability, the report said.
The Post noted that one key breakthrough that had not been publicly described was Iran’s success in obtaining design information for a device known as a R265 generator.
The device is a hemispherical aluminum shell that is lined with pellets of high explosives and electrically wired so the detonations occur in split-second precision, the report said. The explosions compress a small sphere of enriched uranium or plutonium to trigger a nuclear chain reaction.
Creating such a device is a formidable technical challenge, and Iran needed outside assistance in designing the generator and testing its performance, the paper said.