The US has said a plan agreed between Iran and the UN's nuclear watchdog to clarify Tehran's nuclear programme has "real limitations".
UN inspectors can visit the Arak plant under July's agreement
US envoy to the IAEA, Gregory Schulte, said Iran had manipulated the watchdog as a way to avoid harsher sanctions.
Iran and the IAEA said on Tuesday they had agreed a timeline for implementing the plan, first announced in July.
Western powers suspect Iran of building a nuclear weapons capability. Tehran says its nuclear programme is peaceful.
The UN Security Council has already approved sanctions against Iran, urging it to stop uranium enrichment activities.
Call for more sanctions
Mr Shulte said he was not impressed by Iran's promise of transparency.
"We understand there are real limitations with the plan, including Iran's continued refusal to implement the IAEA's Additional Protocol" he said.
He was referring to an agency allowing inspectors to make spot checks at site not declared to be nuclear.
The talks were described by the IAEA as constructive
"Moreover, Iran's leadership has made clear that implementation of the plan is dependent on no (UN) Security Council action," he added.
The US would nonetheless continue to push for a third round of sanctions, Mr Shulte said.
In July, Iran and the International Atomic Energy Agency announced a two-month arrangement aimed at clearing up outstanding questions and giving the agency better access to nuclear sites.
A timetable for the plan was agreed in Tehran on Tuesday, and will be made public in the next two weeks.
Olli Heinonen, IAEA deputy director, said the latest talks had been "good, constructive".
Since July's agreement, Iran has allowed the IAEA's inspectors to visit its heavy water research reactor at Arak, and has been holding talks with a UN technical team on guidelines for inspecting its uranium enrichment plant at Natanz.
BBC Iran analyst Pam O'Toole says that over the past few months Iran has appeared anxious to demonstrate the transparency of its programme.
One Iranian official recently expressed hope that the West could respond to Iran's co-operation with the IAEA by not pushing for more sanctions.
However, some Western analysts have alleged that Tehran is simply trying to play for time, hoping to delay the imposition of further sanctions while it increases its nuclear know-how.