Western powers accuse Iran of attempting to create nuclear weapons, but Tehran says it is producing nuclear fuel only for civil nuclear energy.
The IAEA said as long as Iran continued to withhold documentation, permission to interview relevant Iranian officials and visits to sites in question, it would be unable to verify whether Iranian nuclear activity was peaceful or not.
Progress in the IAEA inquiry, which Iran regards as unjust and driven by pressure from the United States, looks unlikely before Iran sees what Barack Obama, the new US president, has to offer under his stated policy of engagement with foes.
Iran has accumulated 839 kg of low-enriched uranium verified by inspectors and told the IAEA this month it had added another 171 kg.
Nuclear analysts estimate anywhere from 1,000 to 1,700 kg would be needed as a basis for conversion into high-enriched uranium suitable for one bomb, if Iran so chose.
Mohamed ElBaradei, the IAEA director general, said on Tuesday that Iran had not added as many centrifuges recently as it could have and the reason was probably political rather than technical.
ElBaradei appeared to be alluding to perceptions Iran may want to give Obama political cover for talks, not provoke harsher UN sanctions over its refusal to suspend enrichment.
In a separate development, the IAEA has said that uranium found at the site of an alleged illicit nuclear facility in Syria last year is unlikely to have come from Israeli missiles that bombed it.
The IAEA's "current assessment is that there is a low probability that the uranium was introduced by the use of missiles," the agency said in a restricted report obtained by the AFP news agency.
Syria has alleged that particles of uranium found at a remote desert site which the United States alleges was a covert nuclear reactor came from the Israeli bombs that razed the site in September 2007.
Gordon Duguid, a US state department spokesman, said Washington expected the "mounting evidence and ongoing concerns" to be addressed by the IAEA board of governors at its next meeting from March 2 to March 6.
"We fully support the IAEA in its investigation and urge the international community to continue insisting that Syria comply with its IAEA obligations and cooperate fully with the IAEA without delay," he said.
A senior IAEA official, speaking on condition of anonymity, said "it's nuclear material that hasn't been declared and Syria has to explain" how it got there.