UN nuclear inspectors have found traces of uranium enriched at 27% at Iran's Fordo nuclear site, a confidential report obtained by the BBC suggests.
The report by the nuclear watchdog IAEA comes a day after Iran and world powers held talks on Iran's nuclear programme.
Iran says the aim of the Fordo site is to enrich uranium for civilian use up to a maximum of 20%, and the latest readings could be accidental.
Analysts say 27% would bring Iran closer to making weapons-grade uranium.
The latest report by the International Atomic Energy Agency says Iranian officials told inspectors that the production of such particles "may happen for technical reasons beyond the operator's control".
Iran insists its nuclear work is purely peaceful, but Western countries fear it is seeking nuclear weapons, which require uranium enriched up to 90%.
After the latest high-level talks in Baghdad, EU foreign policy chief Catherine Ashton said "significant differences" remained, but some common ground had been found.
More talks between Iran and six powers - Britain, the US, China, Russia, France and Germany - are due to be held in Moscow next month.
The goal of the international team, led Mrs Ashton, is an agreement by Iran to curb uranium enrichment and allow UN inspectors to verify its nuclear activity is peaceful.
The previous series of full international talks with Iran broke down in early 2011.
Last November an IAEA report claimed Iran had "carried out activities relevant to the development of a nuclear device".
In its latest report, the Vienna-based agency reaffirms that it is seeking access to the Parchin military site.
The IAEA says that since November it has obtained more information relating to possible high-explosive tests there, which could be relevant to nuclear warheads.----
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