The United Nations nuclear agency says it needs more clarification about Iran's previously undeclared uranium enrichment site near the city of Qom.
In a report, the IAEA says the delay in admitting the plant's existence raises concern about other possible secret sites, the BBC reports.
A United Nations team was allowed to inspect the facility this month, but believes it has been in operation for longer than Iranian officials have claimed.
The IAEA report also notes that the number of centrifuges in use at the main uranium enrichment plant in Natanz has been reduced.
Some Western nations fear Iran wants to develop nuclear weapons, but Tehran maintains its nuclear programme is peaceful.