Mohamed ElBaradei, director general of the International Atomic Energy Agency, spoke as its 35-nation board of governors gathered for a meeting to review Iran's record of working with the agency.
"We still have a central issue and that is whether Iran has declared all its enrichment activities," he said.
An Iran resolution calling for increased cooperation and disclosure is likely to be presented at the meeting, which will also review an agency report noting that Iran granted IAEA inspectors access to sites but otherwise failed to eliminate international concern that it is attempting to make nuclear weapons.
But any resolution is unlikely before midweek at the meeting, which could run through the week and will also review the scrapping of Libya's nuclear program. That gives Iran time to press for weakened language in any text — and time for the United States and its allies to try to toughen the wording.
The agency is chiefly concerned with two issues: contradictory, missing or withheld information on the scope of Iran's enrichment program, and the source of enriched uranium found inside the country.
"These are two issues where we need accelerated and proactive cooperation," ElBaradei told reporters before the meeting opened Monday. "The way they have been engaging us on this issue has been less than satisfactory, and I'm calling on them and expect the board also to continue to call on them to become more transparent."
In separate comments inside the meeting, ElBaradei said Iran's responses to IAEA requests about its centrifuge enrichment program have been "changing and at times contradictory."
While the Iran resolution will not directly threaten sanctions, any toughly worded document will maintain pressure on Iran to come clean on still-blurry aspects of what was a covert nuclear program for nearly 20 years until discovered two years ago.
Iran has denied being uncooperative, and has rejected U.S. allegations that its nuclear program is for military purposes. Instead, the country says its uranium-enrichment program — which could be used to make bombs, once fully operational — is geared solely toward generating electricity.
ElBaradei said last month thatof a concrete link between Iran's nuclear activities and its military program, but that "it was premature to make a judgment."
The IAEA report, written by ElBaradei, says Iran inquired about buying thousands of magnets for centrifuges on the black market — casting doubt on Iranian assertions that its P-2 centrifuge program was purely experimental and not geared toward full uranium enrichment.
On the traces of enriched uranium — which include minute amounts at weapons-grade levels — Tehran says they were not domestically produced but inadvertently imported in purchases through the nuclear black market.
But IAEA investigators have not been able to fully test that claim because Pakistan, the main source of the equipment, has blocked free access to its nuclear material, meaning the agency cannot match isotope samples to the traces found in Iran.
Sayed Hossein Mousavian, a senior official of Iran's powerful Supreme National Security Council, suggested that those two issues were close to being cleared up. He told reporters in Vienna that his country had submitted new information on the P-2 centrifuges, and asserted that all but one case of enriched uranium contamination had been explained.
But several Western diplomats said the new information was still being assessed and flatly denied that the contamination issue had been reduced to one unresolved case.
Kenneth Brill, the chief U.S. delegate to the meeting, said ElBaradei's comments amounted to a "very firm message that Iran needed to do much better than it has been doing."
Under pressure since the start of international scrutiny, Iran has suspended uranium enrichment and stopped building centrifuges. It also has allowed IAEA inspections of its nuclear facilities without notice.
The meeting will likely be asked to accept some form of a tough draft resolution, written by France, Britain and Germany, urging Iran to halt operations of a plant inaugurated in March that processes uranium into gas. The draft also calls for aborting plans to build a heavy water reactor.
Mousavian, the senior Iranian official, said the language of the draft, as it now stood, was "counterproductive."
A diplomat, who like the others spoke on condition of anonymity, said small changes had been made to the draft on Friday — including the introduction of a "time element" that nonetheless falls short of imposing a deadline for Iran to come clean. It also left out a "trigger mechanism," wanted by the United States, that would allow additional pressure if Tehran failed to satisfy board demands within a given time.
ElBaradei suggested that a deadline was not necessary but urged Iran to clear up all open questions "in the next few months."
"It is really essential," he said. "We can't go on forever."