Iran voiced defiance yesterday in the face of western condemnation over a new nuclear fuel plant, with a senior official saying the facility would soon be operational and make the "enemies blind".

US president Barack Obama demanded on Friday that Iran come clean about its nuclear programme or risk "sanctions that bite" after the disclosure of the new uranium enrichment plant under construction southwest of Tehran.

Yesterday, Obama said the discovery of the facility showed a "disturbing pattern" of evasion by Tehran which added urgency to its planned talks with world powers in Geneva on Thursday.

The west accuses Iran of seeking to acquire a nuclear weapon. Tehran insists its nuclear activities are aimed at generating electricity so that it can export more oil and gas.

Iran acknowledged the existence of the uranium enrichment facility near the holy city of Qom for the first time on Monday in a letter to the UN International Atomic Energy Agency.

US officials said the disclosure was aimed at pre-empting an announcement by western governments, which were aware of the site, but Iranian president Mahmoud Ahmadinejad said the facility was legal and open for inspection by the IAEA.

"This new plant, God willing, will soon become operational and will make the enemies blind," the semi-official Fars News Agency quoted Mohammad Mohammadi- Golpayegani, who heads the office of Supreme Leader Ayatollah Ali Khamenei, as saying.

On Friday, Iran's president Mahmoud Ahmadinejad said the facility was around 18 months from beginning operations and that western powers would regret accusing Iran of hiding it. Iran already has a uranium enrichment plant near the central city of Natanz.

Mohammadi-Golpayegani, a cleric who was speaking at a ceremony marking the start of the 1980-'88 Iran-Iraq war, said the construction of the facility was a sign Iran was at the "summit of power", Fars reported.

Obama, British prime minister Gordon Brown and French president Nicolas Sarkozy, in Pittsburgh for a Group of 20 summit, made a joint appearance on Friday to level the new charges against Iran over its nuclear programme.

Obama said Tehran had been building the nuclear plant in secret for years and urged it to address international concern that its nuclear programme is geared towards making bombs.

"This is a serious challenge to the global non-proliferation regime and continues a disturbing pattern of Iranian evasion," Obama said.