UN Secretary-General Ban Ki-moon voiced serious concern in a meeting with Iran's president on Friday that Tehran was building a new uranium enrichment facility.

(Ban) expressed his 'grave concern about its (Iran's) activities related to continued uranium enrichment as demonstrated by the construction of a new uranium enrichment facility,' Ban's press office said in a statement.

'He emphasized that the burden of proof is on Iran.'

'The Secretary-General reiterated his call on Iran to fully implement relevant Security Council resolutions as well as fully co-operate with the IAEA (International Atomic Energy Agency) towards resolving all outstanding concerns related to its nuclear program,' the statement added.

US President Barack Obama demanded on Friday that Iran come clean about its nuclear program or face 'sanctions that have bite' after the disclosure of the secret uranium enrichment plant.

The fresh disclosure of the scope of Iran's disputed nuclear program added a new sense of urgency to Tehran's much-anticipated talks with the United States and five other powers next Thursday in Geneva.

Iran has maintained its nuclear program is strictly for peaceful electricity generation. A defiant Iranian President Mahmoud Ahmadinejad insisted the facility near the holy city of Qom was legal and open for inspection by the IAEA, the UN nuclear watchdog.

He said the plant was 18 months from starting operations and Western powers would regret accusing Iran of hiding it.

'It's not a secret site,' Ahmadinejad told a news conference in New York where he was attending the UN General Assembly.

Ban also 'expressed his continuing concern about the human rights situation in Iran, with respect to freedoms of association, assembly and practice of religion,' his press office added.

'He (Ban) underlined the need to uphold due process and transparency in the trials and treatment of post-elections and other detainees,' the statement said.-Reuters