UN nuclear inspectors have found traces of uranium enriched at 27% at Iran's Fordo nuclear site.
The announcement by nuclear watchdog IAEA comes a day after Iran and six world powers held talks on Tehran's controversial 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 IAEA has requested further details.
According to the IAEA report, the Iranians 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%. Read More