The allies will demand that Iran prove to the increasingly skeptical group that its intentions with its various sites are peaceful and energy-related, as Iran claims, and not for weapons development, as the West believes, the official said Saturday.
These nations now agree that they are less inclined to listen to suspect arguments or incomplete evidence - viewing it as a stall tactic, the official said.
But beyond the timeframe of "weeks" for coming clean on Qom, the allies will not give Iran a specific deadline to provide the information about its overall program, the official said.
The development of such a timeframe will depend on the Iranians' actions in the meeting and directly after it, the official said.
The kind of transparency the group wants from Tehran is far-reaching, covering people, timeframes and facilities. This would include full access for the International Atomic Energy Agency to any and every site, notebooks, computers and documents related to nuclear development, and all scientists.
The United States will be represented in Thursday's meeting in Geneva by William Burns, the under secretary of state for political affairs, a career diplomat.
On Saturday, Iran's nuclear chief told state TV that his country would allow the U.N. nuclear agency to inspect Iran's newly revealed and still unfinished uranium enrichment facility. Ali Akbar Salehi didn't specify when inspectors from the IAEA could visit. He said the timing would be worked out with the U.N. watchdog.
Earlier Saturday, President Barack Obama offered Iran "a serious, meaningful dialogue" over its disputed nuclear program, while warning Tehran of grave consequences from a united global front.
"Iran's leaders must now choose - they can live up to their responsibilities and achieve integration with the community of nations. Or they will face increased pressure and isolation, and deny opportunity to their own people," Obama said in his radio and Internet address Saturday.
The White House responded to the development by urging Iran's complete and immediate cooperation with the IAEA. "After hiding this site from the international community for years, full transparency is essential, and it is time for Iran to play by the rules like everyone else," White House spokesman Tommy Vietor said.
Obama said in his address that evidence of Iran's building the underground plant "continues a disturbing pattern of Iranian evasion" that jeopardizes global nonproliferation.
Ahead of Thursday's international talks with Iran in Geneva, Obama said the world "is more united than ever before" on this issue. Those negotiations, he said, "now take on added urgency." Iran's failure to comply with international inspectors raised the potential of tougher economic penalties, although Obama and administration officials did not rule out military action.
"My offer of a serious, meaningful dialogue to resolve this issue remains open," Obama said, urging Tehran to "take action to demonstrate its peaceful intentions."
Evidence of the clandestine facility was presented Friday by Obama and the leaders of Britain and France at the G-20 economic summit in Pittsburgh. The news overshadowed developments on regulating financial markets and reducing fossil fuel subsidies.
Soon after, Russian President Dmitry Medvedev, at his own news conference, urged Iran to cooperate, as did Chinese Vice Foreign Minister He Yafei. He, however, did not endorse penalties against Tehran.
At a news conference in New York, Iranian President Mahmoud Ahmadinejad said his country had done nothing wrong and Obama would regret his actions.
"What we did was completely legal, according to the law. We have informed the agency, the agency will come and take a look and produce a report and it's nothing new," he said.
Ahmadinejad said the plant - which Iranian officials say was reported to nuclear authorities as required - wouldn't be operational for 18 months. But he sidestepped a question about whether Iran had sufficient uranium to manufacture a nuclear weapon.
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