As the watchdog, the International Atomic Energy Agency, began a board of governors meeting in Vienna, IAEA chief Mohamed ElBaradei told reporters that both Iran and Libya "have been in breach of their obligations under the safeguards agreement" of the nuclear Non-Proliferation Treaty (NPT).
"In view of many years of violation of non-proliferation obligations by Libya and Iran, I am asking for the provision of information and a full measure of transparency," he said.
Responding to Tehran's call Sunday for the IAEA to close its file on Iran's nuclear programme, ElBaradei said "that depends very much ... on the kind of cooperation we hopefully will continue to receive from Iran."
The United States charges that Iran is secretly developing nuclear weapons, but Tehran maintains its nuclear program is strictly peaceful.
US ambassador to the IAEA Kenneth Brill said Monday that Iran was changing its story in order to hide a continuing nuclear weapons program from investigation by the IAEA.
"The fact is the Iranians change their stories to fit the facts," Brill said.
Diplomats said the United States was trying to get the IAEA board to adopt a tough resolution condemning the Iranians for hiding sensitive technology and insisting on a "trigger mechanism" for the matter to go before the UN Security Council, which could impose sanctions, if more violations are discovered.
But Europeans and other countries, led by Britain, France and Germany, resist this as they want to encourage Iran's cooperation with the IAEA.
The so-called Euro 3 had struck an agreement in October for Iran to cooperate with the IAEA. The three states advocate a tactic of "constructive engagement" with the Islamic Republic.
"The US text is not balanced. Washington acts as if nothing positive has happened since November" despite Iran's allowing wider inspections by the IAEA, one diplomat said.
ElBaradei told the 35-nation IAEA board that he was "seriously concerned that Iran's October declaration did not include any reference to its possession of P-2 centrifuge designs (for making enriched uranium which could be weapon-grade) and related RD (research and development) which in my view was a setback to Iran's stated policy of transparency."
ElBaradei said "this was particularly the case" since the October declaration was trumpeted by Iran as providing "the full scope of Iranian nuclear activities" including "a complete centrifuge R and D chronology."
But Iran's ambassador to the IAEA Pirooz Hosseini told reporters Tehran did not say everything in a report last October "because what we announced at that time was based on our obligations under the safeguard agreement" of the NPT.
In December, Iran signed an additional protocol to the NPT allowing for wider inspections and Hosseini said that "according to the time line of the additional protocol, we are going to provide every information which is necessary for the agency."
Brill said senior Iranian officals had said the October report would be "full, complete and represent total transparency."
"When it was proved that was not the case, that the Iranian declaration was neither correct nor complete in very significant ways" then the Iranians "changed their story and said we didn't mean it was going to be full and complete," Brill said.
The IAEA board is expected to report Libya to the UN Security Council for previous non-compliance with the NPT but praise it for its current cooperation in eliminating weapons development programs.
A Western diplomat said that Libya was "almost done in revealing its nuclear program" and the non-compliance declaration would merely be a "pro-forma" way of closing the chapter with no threat of sanctions against the North African country.