Many suspicions still hang over Iran's apparently peaceful nuclear programme, according to a new report by the UN's atomic watchdog, the IAEA.
International pressure made Iran open up to nuclear inspections
The report credited Iran with opening up its nuclear programme to inspections but said key issues were unresolved.
It said the discovery of bomb-grade uranium traces and an Iranian bid to buy centrifuges were of concern.
Iran's top security official, Hassan Rowhani, said the concerns were "minor points" and nothing important.
Mr Rowhani said Iran had no secret nuclear activities and there were no major issues to deal with.
The IAEA has been checking Iran's insistence that its atomic capability is purely for peaceful purposes.
Suspicions that Tehran might be trying to build a nuclear bomb were first raised nearly two years ago.
Under international pressure, Iran agreed to open its doors to UN inspectors and stop uranium enrichment and related activities.
But delays have dogged the inspections process, prompting the US to claim Iran is trying to conceal the military character of its programme.
The IAEA report was prepared in advance of a 14 June meeting of the watchdog's governors, who are expected to debate policy towards Iran.
According to the IAEA, Iran has admitted to buying key components, such as magnets, for use in sophisticated centrifuges from abroad.
This apparently contradicts earlier Iranian claims that parts for the special centrifuges, which speed up the enrichment of uranium, were domestically manufactured.
According to the report, Iran also used a private intermediary to inquire about importing several thousand key components for the centrifuges - again casting doubt on whether the programme was for "research and development" alone.
Iran said that magnets had been bought but that they were for other industrial uses.
The head of the country's nuclear programme also confirmed three private companies were still producing centrifuge components.
Mr Rowhani said this was because agreement had not yet been reached on compensation for suspending activities.
In an interview given to the Associated Press agency before the IAEA report became public, US State Department official John R Bolton accused Iran of engaging in "denial and deception" and called for the UN Security Council to rule on the matter.
'Jury still out'
The IAEA also voiced concern at the discovery of fresh traces of highly-enriched weapons-grade uranium.
The uranium, enriched to 36 percent, is well beyond the levels needed for peaceful energy production.
It questioned the Iranian claim that these traces were on equipment bought from abroad.
However, the IAEA chief Mohamed ElBaradei warned that it was too early to jump to conclusions.
"The jury is out on whether the programme has been dedicated exclusively for peaceful purposes," he said.
Mr Rowhani, who has responsibility for Iran's nuclear programme, said the IAEA report contained "certain faults and certain shortcomings".
"The agency has gone into questions that do not concern it," he said.
"We will respond to them, but when we read the report, we see that there are only minor points and nothing important."