At an international conference in Washington marking the 60th anniversary of the first official non-proliferation proposal, Mr ElBaradei said Iran 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," Mr ElBaradei said in a speech at the Carnegie International Non-Proliferation Conference.
"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," a transcript of Mr ElBaradei’s speech said.
Nevertheless, he said he was heartened by Iran’s recent moves toward greater openness.
"We are making progress," Mr ElBaradei said. "We are getting access." But he added: "The ball is in their court"
The UN's nuclear watchdog chief is due to meet US Secretary of State Condoleezza Rice on Tuesday.
Ms Rice has said the US -- which views Iran as a state sponsor of terrorism -- is opposed to Iran obtaining nuclear weapons.
Mr ElBaradei's remarks come a day after Iran formally asked Britain, France and Germany to reopen nuclear talks, which broke off in August when Iran, under new hardline president Mahmoud Ahmadinejad, resumed uranium conversion in defiance of international calls for a suspension to be upheld.
Iranian officials have said the Islamic Republic would convert a fresh batch of uranium ore, rejecting EU calls for a renewed freeze on such activities.
Iran maintains that it is only developing nuclear energy but its insistence on converting uranium, a precursor to enrichment, has raised fears it’s trying to make an atomic bomb.
Sanctions not ruled out
The EU has not ruled out imposing sanctions on Iran over its disputed nuclear program, but said it was studying Tehran’s offer to resume talks first.
British Foreign Secretary Jack Straw, one of three EU ministers leading efforts to engage Iran by offering benefits in return for pledges on its nuclear plans, said trade and diplomatic sanctions were not yet an issue.
"That has not been discussed so far, I'm not going to speculate on the future," he told reporters in Brussels.
But Mr Straw underlined the need for Iran to respond "positively" to a resolution by the UN nuclear watchdog.
"The Iranians are under the obligation to respond positively to the resolution of the board of governors at the International Atomic Energy Agency in late September and we look to them to do that," he said.
Mr Straw confirmed that "informal discussions" were continuing with the Iranians, despite the deadlock on the formal nuclear talks.
The IAEA is to meet on November 24 and could theoretically refer Iran to the UN Security Council for sanctions.
The EU-3 has attempted to persuade Iran to permanently suspend uranium enrichment as a watertight guarantee that its nuclear programme is peaceful.
But Iran insists its rights are enshrined in the nuclear Non-Proliferation Treaty (NPT) and its additional protocol, of which it is a signatory.
Previous attempts to take Iran before the UN Security Council have foundered over Russian opposition and Moscow is once again expected to play a key role.
Iran's Foreign Minister Manouchehr Mottaki on Monday reaffirmed his country's claim that its programme is peaceful and legal.
"We insist on Iran's undeniable right in the (NPT) and international law, which is our right to peaceful nuclear technology," said the minister.
Iran refuses to go back on conversion and says it is only ready to negotiate as long as its right to enrichment is recognised.