The UN nuclear watchdog chief, Mohammed ElBaradei, has accused Iran of dragging its feet in disclosing relevant information about its controversial nuclear programme.
Mr ElBaradei, who heads the International Atomic Energy Agency (IAEA), said that his agency was particularly concerned about Iran's uranium enrichment programme.
"Iran has been offering us additional information, additional access but not the 100% transparency and not the pro-active co-operation I would like to see," said Mr ElBaradei, before flying from Frankfurt to Tehran for further talks on the issue.
The IAEA has set a deadline of 31 October for Iran to prove it is not trying to produce nuclear weapons, and - if doubts still remain - the issue could be referred to the UN Security Council.
The United States has accuses Iran of trying to secretly develop nuclear weapons, but Tehran insists its nuclear programme is aimed at developing nuclear energy for peaceful purposes.
In a statement, the IAEA said the aim of Mr ElBaradei's talks in Tehran was "to clarify important questions that are still outstanding about Iran's nuclear programme".
Mr ElBaradei said Iran had provided some information but added that the core issue for the agency was understanding the nature of Iran's uranium programme.
"We have been making progress but not with the speed we would like to see," he said.
Mr ElBaradei again stressed that Iran must deliver the information before the deadline expires, saying that it was a task which should not take more than a week or two.
"I am ready obviously to walk the extra mile if we can bring the issue to closure as soon as possible," he added.
A source close to the agency said inspectors recently visited a facility that was part of a military complex which was revealed by an Iranian opposition group in July, the BBC's Bethany Bell at the IAEA's headquarters in Vienna says.
On previous, arranged, inspections to Iranian facilities, inspectors found traces of highly enriched uranium - sparking suspicions that the country was seeking to build a nuclear bomb.
Enriched uranium can be used to fuel nuclear power stations, but if enriched further, it can also be used in weapons.
Tehran has also not accepted another key condition which the IAEA is seeking to impose - the right of UN inspectors to mount snap inspections.
Iran hopes that its main nuclear complex - Bushehr plant - will become operational in 2005.
Russia has been helping Iran to complete the $800m project despite strong pressure from Washington.