"Time is indeed running out," said Mark Gwozdecky, spokesman for the Vienna-based International Atomic Energy Agency (IAEA).
"The Iranian authorities have provided some information to date, and have promised more in the future. However, the information is not coming with the speed which we need. It is not yet 100 percent," Gwozdecky said.
The IAEA's board of governors on September 12 gave Iran until the end of October to produce a detailed list of its nuclear-related equipment, cease uranium enrichment and sign an additional protocol to the nuclear Non-Proliferation Treaty (NPT) allowing tougher inspections.
Iran has said it is not bound by the resolution -- passed after heavy US lobbying -- but will nevertheless try to answer the IAEA's concerns.
"In terms of inspections, so far we have been allowed to visit those sites to which we have requested access," Gwozdecky told AFP.
"The core issue for us is understanding the history and nature of Iran's uranium enrichment programme. In particular we need to determine whether or not Iran has had nuclear activities which were not declared to the IAEA.
"This means that no later than October 31, Iran must provide full and complete information on their nuclear programme. This task is certainly do-able in this timeframe and really shouldn't take more than a week or two," Gwozdecky said.
The UN spokesman said however that Iran had "provided some information on their equipment imports" though more information was needed on the origin of these components.
"There are two phases to our work. The first phase involves Iran providing all information to us on the unresolved questions, not later than October 31. The next phase is our verifying the accuracy of this information. This task may require additional time beyond the date of October 31."
A failure by Iran to meet the IAEA deadline could see it being declared in violation of the NPT and the matter being passed to the UN Security Council, which could impose sanctions.