In a statement to the board of the International Atomic Energy Agency, Mohamed ElBaradei said Iran has been showing increased cooperation, but that his experts still don't have enough information to determine the nature of Tehran's nuclear activities.
"I would urge Iran in the coming weeks to show proactive and accelerated cooperation, and to demonstrate full transparency by providing the agency with a complete and accurate declaration of all its nuclear activities," ElBaradei said.
The IAEA's 35-nation board of governors, meeting Monday at the agency's Vienna headquarters, is expected this week to urge Iran to make its nuclear program accessible by agreeing to allow more intrusive inspections without notice.
Iran, however, has been warning the United States and others not to push for too much too soon, warning that an ultimatum could heighten tensions.
The United States accuses Iran of working on a secret nuclear weapons program, and a recent confidential IAEA report, obtained by The Associated Press, said traces of highly enriched, weapons-grade uranium were found at an Iranian nuclear facility.
The report also said Iran was conducting tests that experts say make little sense unless the country was pursuing nuclear weapons.
Tehran insists its nuclear programs are for generating electricity and says its equipment was "contaminated" with enriched uranium by a previous owner.
U.S. spokesmen have questioned why an oil-rich nation like Iran would need such an aggressive nuclear energy program.
ElBaradei said the IAEA was pressing the Iranians for a complete list of all imported equipment and components they contend were contaminated as well as its origins, the dates it was acquired and where it has been used or stored since.
Only a comprehensive declaration can determine the truth, ElBaradei said. Some of the information Iran recently has handed over is "piecemeal" or "inconsistent with that given previously," he said without elaborating.
"Iran should move rapidly" toward signing the measure allowing more rigorous inspections, ElBaradei said, adding: "The more transparency that is provided, the more assurance we can give."
The outcome, he told reporters, "will have major implications for the nonproliferation regime" worldwide.
In Iran, government spokesman Abdollah Ramezanzadeh said Monday that his country would "act in accordance with our obligations."
"In the past year, especially in the past two months, we have extended the necessary cooperation with the IAEA," he said. "IAEA inspectors have visited the locations they demanded to visit."
Iran's delegate, Ali Akbar Salehi, said before the meeting that his country remained open to negotiating the inspection issue with the IAEA, but he indicated the offer could be withdrawn if this week's board review "disrupted the whole process."
"We are sitting on a very thin edge," Salehi said. "It could tilt one way or the other very easily."
The nuclear agency also needs to know more about Iran's uranium conversion experiments and its testing of gas centrifuges to enrich uranium, he said.
"It is essential that all outstanding issues, particularly those involving high-enriched uranium, be brought to closure as soon as possible to enable the agency to come to a definitive conclusion," he said.
Suspicions about Iran's nuclear activities prompted ElBaradei in February to tour Iran's nuclear facilities, including the incomplete plant in Natanz, about 300 miles south of Tehran.
Diplomats said he was taken aback by the advanced stage of a project using hundreds of centrifuges to enrich uranium.
In an apparent victory for Iran, the Bush administration last week decided not to ask the Vienna meeting to endorse a resolution finding Iran in noncompliance of IAEA obligations.
That could bring the matter before the U.N. Security Council, which can take steps ranging from criticism to sanctions. "There was no other choice but to back down, because that proposal didn't have many countries to go along with it," Salehi said Monday.
Instead, the resolution being drafted likely will call on Iran to answer questions raised in the report and provide full disclosure of its program. It also could set a deadline for Tehran to comply, diplomats said on condition of anonymity.
Kenneth Brill, the chief U.S. delegate to the IAEA, declined to comment on what the Americans were seeking. But he said the United States and others believe Iran is trying "to evade international obligations and to seek the capacity to build nuclear weapons."
"It's fair to say that the majority of board members will want to see Iran … enhance its cooperation" and "provide the answers to all the questions that are outstanding," he said.
Iran was listed as part of the "axis of evil" with North Korea and Iraq in President's Bush's January 2002 State of the Union speech.
Tensions have risen during the Iraq war as Washington accused Iran of dispatching agents to cross the border and sow dissent among Iraqi Shiites. Some hard-line advisers to the Bush administration advocate a policy of regime change towards Iran similar to the approach to Iraq, which ended with Saddam Hussein's ouster.
Iran, for its part, protested a brief truce that American forces made with a terrorist group called the Mujahadeen al Khalq, which opposed the regime in Tehran.