The United States has again accused the United Nations nuclear watchdog of damaging its credibility in its latest report on Iran's nuclear activities.
Iran denies it is developing nuclear weapons
The US envoy to the International Atomic Energy Agency (IAEA) questioned findings that there was no evidence of an ongoing nuclear programme in Iran.
But IAEA Director General Mohammed ElBaradei has defended the report.
A key meeting of the agency's board of governors considering the report has been adjourned until Wednesday.
The IAEA report detailed 18 years of concealment by Iran, but failed to find evidence of the secret nuclear weapons programme which the US has accused Iran of pursuing.
Speaking at an agency board meeting, the US ambassador to the IAEA, Kenneth Brill, said the report's assertion was "questionable".
He said it was difficult to believe that Iran had now come clean given its past history of deception.
Mr ElBaradei said work was still in progress on verifying Iran's nuclear programme, but that he found American scepticism about the failure to find evidence of a nuclear weapons programme "disingenuous".
"We reflect facts, as radar does, without partiality," he said, according to Associated Press news agency.
"We do not jump to conclusions or make leaps of faith. We have not said that we have come to the conclusion that the Iranian program is exclusively for peaceful purposes, because we still have work to do."
US negotiators appear to have dropped demands that Iran be reported to the UN Security Council.
This could have led to the imposition of sanctions on Iran.
Mr ElBaradei earlier said the IAEA would reconvene its meeting next Wednesday, with only the issue of the response to be resolved.
"By that time we will have a draft resolution hopefully to be adopted, I still believe, by consensus," he said.
He added that the board had formally approved Iran's intention to sign up to tougher inspections of its nuclear programme.
In his report, Mr ElBaradei said that Iran had secretly produced plutonium and enriched uranium - materials which can be used in nuclear weapons.
The US has urged the agency to declare Tehran in non-compliance with the nuclear Non-Proliferation Treaty (NPT).
But Iran has indicated it may reconsider signing this additional protocol if such a resolution is issued.
Iran has admitted to some violations in the past but says its nuclear programme is peaceful.
Britain, France and Germany have reportedly been proposing that the IAEA "strongly deplore Iran's past breaches", after an earlier draft seeking Iran's continued co-operation with the IAEA was criticised by the US as too weak.