The head of the International Atomic Energy Agency spoke as European negotiators studied a new Iranian offer to resume talks over its disputed nuclear programme, while the Islamic Republic also moved toward more work on uranium enriched fuel.
In remarks prepared for an international conference here for the 60th anniversary of the first official non-proliferation proposal, ElBaradei said Tehran must come clean on its nuclear intentions.
"Over the past two and half years, we have compiled a detailed picture of most aspects of Iran's past and current nuclear program," ElBaradei said, according to the transcript.
"But given that the program was concealed for nearly 20 years, and that a number of open questions remain, the responsibility rests with Iran to provide, if needed, additional transparency measures," ElBaradei said.
ElBaradei added however that he was heartened by some recent moves toward greater openness. "We are making progress," he said. "We are getting access."
But he insisted: "The ball is in their court"
The UN's nuclear watchdog chief, winner of this year's Nobel Peace Prize, is to meet US Secretary of State Condoleezza Rice Tuesday.
The United States - which views Iran as a state sponsor of terrorism - has long accused the Islamic state of seeking to develop nuclear weapons, although it has backed European efforts to engage Tehran.
Iran insists that its nuclear program is peaceful and that it has the right to civilian nuclear energy.
ElBaradei said that improved transparency from Tehran would "enable the (UN) agency to resolve these questions, and to provide the required assurance about the peaceful nature of Iran's nuclear program."