The U.N.'s International Atomic Energy Agency has found evidence of uranium enriched up to levels closer to the threshold needed for developing nuclear warheads, at an underground Iranian facility.

Diplomats, under the condition of anonymity, said that uranium had been enriched up to 27 percent -- still far below the 90 percent threshold for weapons-grade material -- at Iran's Fordo enrichment site beneath the mountains southwest of Tehran.

The diplomats clarified that the more highly-enriched uranium did not necessarily mean that Iran is attempting to develop weapons-grade material, but that it was more likely that Iranian scientists had miscalibrated their centrifuges, leading to over-enriched uranium.

Iran is currently under sanctions from the U.S. and E.U. over its nuclear program. Recent talks in Baghdad were stalled over demands for Iran to curb uranium enrichment before lifting of sanctions could be considered, an offer which Iran rejected, instead offering to allow U.N. inspectors access to its nuclear facilities in exchange for an end to the sanctions.

Iran is scheduled to resume talks with the six-nation negotiating bloc -- Britain, China, France, Germany, Russia and the U.S. -- in Moscow next month.