The "ball is very much in Iran's court on this issue", said ElBaradei, head of International Atomic Energy Agency.
"We need a number of additional transparency measures," he said on Monday, pressing Iran to provide "access to certain sites, including access to certain individuals, including making a number of documents available".
As the IAEA meets this week in Vienna, the US and the European Union are calling for the agency to refer Iran to the Security Council for sanctions over two decades of concealing atomic activities and a resumption of nuclear fuel work last month.
The crisis escalated at the UN General Assembly on Saturday when Iranian President Mahmoud Ahmadinejad attacked Western demands he halt uranium enrichment activities, which can produce fuel for nuclear power reactors but also the raw material for atom bombs.
EU negotiators Britain, France and Germany appear determined to do something about Iran's intransigence, even though there is stiff opposition on the IAEA's 35-nation board of governors to referring the matter to the Security Council.
The Security Council could use measures ranging from resolutions to trade sanctions to try to get Tehran to stop making nuclear reactor fuel.
Iran is accused of hiding an
atomic weapons programme
ElBaradei took heart from breakthrough agreement by North Korea, which promised on Monday to give up its nuclear weapons in exchange for pledges of aid and security.
The deal was reached in talks between North Korea, South Korea, China, Japan, Russia and the US.
"What we have seen coming today from North Korea gives me a lot of encouragement that issues as complex (as Iran) can in fact be resolved if good faith on both sides exist," ElBaradei said.
IAEA inspectors should return to North Korea as early as possible after having been kicked out in December 2002, he said.
On Iran, "regrettably we are going through a period of confrontation and political brinkmanship", ElBaradei said.
Peace, peace and peace
The IAEA has been investigating Iran since February 2003 on US charges that the Islamic Republic is using what it says is a peaceful nuclear power programme to hide atomic weapons work.
Iranian ambassador Mohammad Akhondzadeh said on Monday that the "intention of Iran is peace, peace and peace".
The Western drive for UN action against Iran's nuclear fuel programme faces opposition from Russia and China, which both have major business interests with Iran, as well as non-aligned states.
These countries back Iran's claim to peaceful nuclear technology under the nuclear Non-Proliferation Treaty (NPT).
"What good does it do to refer to the Security Council with 15 abstentions? This is terrible, even 10 abstentions"
The IAEA board last month called on Iran to halt uranium conversion it had resumed in August.
Iran had suspended fuel work in November to start talks with the European trio on guaranteeing Iran's nuclear programme is peaceful.
Conversion is the first step in enriching uranium for reactor fuel.
"The Iranians are convinced they are in a strong position" as the United States is bogged down militarily in Iraq and the Islamic Republic has clout as a crucial supplier to an already tight world oil market, a non-Western diplomat close to the IAEA said.
While the West has a slim majority to push the matter through the board, diplomats said this would be a disaster for a body that has always approved nuclear compliance matters by consensus.
"You can refer it by vote but then this will show very weak support," especially with nuclear powers Russia and China able to veto any Security Council measures, the diplomat said.
Ahmadinejad has rejected
demands to halt enrichment
"What good does it do to refer to the Security Council with 15 abstentions? This is terrible, even 10 abstentions," the diplomat said, referring to a possible count from the 35-nation board. Some countries might abstain so as to avoid direct opposition to the West.
Diplomats said that the EU3, which has been working on a draft resolution for the past two weeks, might try to gain time by proposing a deadline for Iran to halt the fuel work.
This would give Iran one last chance to comply, with another IAEA board meeting to be called within a few weeks.
In New York on Saturday, US Secretary of State Condoleezza Rice had said there was still "time for diplomacy".