By Caren Bohan and Hossein Jaseb
WASHINGTON/TEHRAN (Reuters) - U.S. President Barack Obama said on Saturday the discovery of a secret nuclear plant in Iran showed a "disturbing pattern" of evasion by Tehran that added urgency to its talks on Thursday with world powers.
Iran's ambassador to the U.N.'s nuclear watchdog said Tehran was arranging for International Atomic Energy Agency inspections at the site "in the very near future," although other Iranian officials struck a defiant note.
One said he hoped Iran's second nuclear enrichment site, under construction southwest of Tehran, would soon be ready to "blind" Iran's enemies.
But Ali Asghar Soltanieh, Iran's ambassador to the IAEA, said he foresaw a quick visit to the site by agency inspectors, although he called the Obama administration's response to the plant's disclosure "discouraging" and a "political show."
U.S. Secretary of State Hillary Clinton welcomed Soltanieh's statement on IAEA inspections, telling reporters, "It is always welcome when Iran makes a decision to comply with the international rules and regulations and particularly with respect to the IAEA."
Iran, which says its nuclear program is aimed at electricity generation rather than weapons production as feared by the West, will meet the United States and five other powers in the Swiss city of Geneva on Thursday.
A senior U.S. administration official said the six powers were preparing "a set of transparency demands" focused on the secret uranium enrichment plant near Qom, a Shi'ite Muslim holy city south of Tehran.
"Those demands include unfettered access for the IAEA to the Qom facility, the people working there, and timelines related to its development," the official said.
"The timeline for this demand for this information about Qom will be weeks."
It is expected the demands will be presented to Iran at the Geneva meeting.
Obama warned Iran on Friday it would face "sanctions that bite" if it did not come clean.
"This is a serious challenge to the global non-proliferation regime and continues a disturbing pattern of Iranian evasion," he said in his weekly radio and Internet address on Saturday.
"That is why international negotiations with Iran scheduled for October 1 now take on added urgency," he said.
Britain, France and Germany have joined the United States in raising the prospect of new sanctions against Iran if it did not take steps to allay concerns about its nuclear program.
Russia also signaled a greater willingness to go along with sanctions, while China said it favored a "dual track" approach of pressure and talks.
U.N. Secretary General Ban Ki-moon expressed "grave concern" in talks with Iranian President Mahmoud Ahmadinejad on Friday and said Tehran had to show its intent was peaceful.
"He emphasized that the burden of proof is on Iran," Ban's press office said in a statement.
Adding to the tension, Iranian media said Iran's elite Revolutionary Guards would hold missile defense exercises starting on Sunday and taking place over several days.
'COMPLETELY STUNNED'
Iran acknowledged it had a uranium enrichment facility near Qom for the first time on Monday in a letter to the IAEA.
The head of Iran's atomic energy organization said Iran thought the disclosure would be welcomed. "We are completely stunned and we were anticipating that the Western countries would welcome this measure by Iran," Ali Akbar Salehi said.
In comments carried by state broadcaster IRIB, Salehi said having two uranium enrichment sites, one near Qom and the other at Natanz, was prudent, given "the unceasing threat of some enemies."
"The Natanz site enjoys complete defensive capability, but the new site excludes any possibility of stopping Iran's peaceful nuclear activity," he said.
U.S. officials said the disclosure was aimed at pre-empting an announcement by Western governments, which were aware of the site. The IAEA demanded immediate access.
Low-enriched uranium can be used as fuel for power plants, while highly enriched uranium can be used to make bombs.
Ahmadinejad said on Friday the plant near Qom was about 18 months away from start-up, legal and open for IAEA inspection and Western powers would regret accusing Iran of hiding it.
"It's not a secret site," he told a news conference in New York where he was attending the U.N. General Assembly.
Salehi said the plant was about 100 km (60 miles) south of Tehran, toward Qom. "There was absolutely no necessity for us to make any announcement on the facility since we are a year-plus before its completion," he said on state television.
Mohammad Mohammadi-Golpayegani, who heads the office of Supreme Leader Ayatollah Ali Khamenei was quoted as saying the plant would start running "soon."
"This new plant, God willing, will soon become operational and will make the enemies blind," the semi-official Fars News Agency quoted him as saying.
The foreign minister of Israel, which has refused to rule out pre-emptive military action to stop Iran developing an atomic weapon, called for an "unequivocal" response.
"The disagreement (on whether Iran is developing military nuclear capability) has been done away with," right-winger Avigdor Lieberman told Israel Radio.
(Additional reporting by Hashem Kalantari and Fredrik Dahl in Tehran, Ari Rabinovitch in Jerusalem, Louis Charbonneau in the United Nations and Ross Colvin in Washington; Editing by Michael Roddy, Peter Cooney and Paul Simao
