Iran denies allegations that it is building a nuclear weapon.
Iran has been asked by the UN's nuclear watchdog, the IAEA, for a substantive response to allegations that it is developing a nuclear weapon.
Without more information from Iran, the IAEA cannot provide assurances about the country's nuclear programme, says the agency's head, Mohamed El Baradei.
Iran says its nuclear programme is purely for peaceful purposes.
Allegations about its nuclear programme are unsubstantiated, says the country's IAEA envoy, Ali Asghar Soltanieh.
Iran has repeatedly denied allegations that it is building a nuclear weapon.
Mr Soltanieh said that Iran had not been allowed to see any of the documents which allegedly back up US accusations of a military nuclear programme.
"How can we question a country without giving authentic documents?" asked Mr Soltanieh.
The IAEA says Iran must provide clarification
"A member state is accused by another member state but that member state has not been given any documents or evidence," he added.
In a report last week, the IAEA said that it had asked Tehran for detailed clarification about its nuclear programme.
However, the agency said that despite those requests, it had not been able clear up questions about Iran's programme.
Speaking to the IAEA's board, Mr El Baradei said Iran should provide "substantive information to support its statements and access to relevant documentations and individuals".
Iran "should clarify the extent to which the documentation is factually correct and where, as it asserts, such information has been fabricated or where it relates to non-nuclear purposes", he added.
Mr El Baradei also said that, so far, tests had failed to find any sign of nuclear material at a site in Syria which was destroyed in an Israeli bombing raid last year.
The US says Syria was building a secret nuclear facility at the site.