Growing pressure on Iran to explain what could be secret nuclear weapons work has left Tehran increasingly defensive — and the U.S. and its allies hopeful they can exploit the situation to wrest concessions from Tehran. But it may be too late for that.

Even as it resists a strong push by the International Atomic Energy Agency for answers to allegations it tried to make nuclear arms, Iran continues to refuse to compromise on the key demand that it stop uranium enrichment.

For years, the Islamic Republic shrugged off offers of economic and political rewards in exchange for an enrichment freeze. It has thumbed its nose both at U.N. Security Council demands that it do so, and at veiled U.S. threats of a military action. Instead, it exploited international indecision and expanded and improved its enrichment capability.

An agency report Monday to the U.N. Security Council and the IAEA board suggested that Tehran was stonewalling investigators and possibly withholding information crucial to the U.N. nuclear monitor's probe of allegations it did nuclear arms research.

A senior U.N. official — who demanded anonymity because he was not authorized to comment on the report — told the AP its tone was unusually tough. And Simon Smith, the chief British delegate to the IAEA, said the language reflected deep "frustration at the lack of cooperation" by Iran.

Briefing board members three days after the report's release, Olli Heinonen — the IAEA's deputy director general in charge of the agency's Iran file — said Iran's possession of nuclear warhead diagrams was "alarming." And diplomats at the closed meeting said Heinonen also left little doubt that in his view, much of the intelligence it had from the U.S. and other board members tended to increase concerns.

John Bolton, who has served as U.S. ambassador to the United Nations and U.S. undersecretary of state in charge of Iran's nuclear file, said the IAEA's new assertiveness should "reactivate debate" within the U.S. government on the use of force against Iran as a last resort.

But Pierre Goldschmidt, Heinonen's predecessor at the IAEA, says force would backfire, giving Iran "justification to seek nuclear weapons."

He suggested the U.N. nuclear watchdog's assertiveness in assessing suspected work on the bomb would have been more effective if the IAEA had received intelligence information in 2003 — when the first evidence of clandestine Iranian nuclear activity surfaced. Instead, the IAEA's 35-nation board, following up less volatile leads, waited until 2006 to send Tehran's nuclear file to the U.N. Security Council.

But "it's never too late," Goldschmidt said. Instead of a military strike, he told The Associated Press that Security Council sanctions more rigorous than the mostly symbolic penalties now in place were needed — along with offers of rewards.

A key incentive? With the Iranians most distrustful of the U.S., talks between Tehran and Washington were the solution, he said.

"I think the Americans should talk to the Iranians directly, bilaterally, multilaterally secretly and (initially) without any preconditions ... (and) at the highest level," he said.

Still, Iran may have less reason now than a year ago to compromise, now that its technicians appear to have eliminated most bugs keeping them from full-scale enrichment expansion.

"In the past, Iran has experienced significant problems" with breakdowns and other technical mishaps keeping it from running its enriching centrifuges smoothly, said David Albright, a former IAEA nuclear inspector. But the newest IAEA findings show "that Iran is overcoming these problems," he added.

Fears that Iran might want to make the bomb are as old as the discovery five years ago that it had assembled the nuts and bolts of a uranium enrichment program.

Enrichment can turn uranium into the fissile material used in nuclear warheads. But it can also be used to generate power and is allowed under the Nuclear Nonproliferation Treaty.

That has allowed Tehran to argue that it has a right to develop enrichment — a stance that resonates in the developing world, where Iran draws much of its support.

But starting last year, the IAEA began focusing on probing for evidence of activities that point more directly to a possible clandestine weapons program.

Based on its own information and intelligence from the U.S. and other board members, it has asked — in vain — for substantive explanations for what seem to be draft plans to refit missiles with nuclear warheads; explosives tests that could be used for a nuclear detonation; military and civilian nuclear links and a drawing showing how to mold uranium metal into the shape of warheads.

Iran remains defiant.

In a statement from its U.N. Mission, Tehran again rejected allegations of an undeclared weapons program as "baseless," "totally false," and aimed at undermining the country's cooperation with the IAEA.

Asked Friday whether the IAEA's new assertiveness was due to U.S. lobbying of the agency, Iranian Foreign Minister Manouchehr Mottaki told the AP that Washington tries "to use any possible capacity as potentiality for their political purposes."

Countries at odds with Washington share such views.

"We don't believe ... Iran has nuclear weapons," Norma M. Goichochea, the chief Cuban delegate to the IAEA, told the AP, when asked about the IAEA findings.

Still, there is evidence that some of Tehran's most important allies might be rethinking their positions.

A diplomat accredited to the IAEA who demanded anonymity for divulging confidential information, told the AP on Friday that China was not opposing a U.S-backed push to introduce a resolution critical of Iran at an IAEA board meeting starting next Monday.

China — along with Russia — has traditionally opposed strong U.N. action against Iran's nuclear program and insisted on watering down the three sets of Security Council sanctions now in place against Iran.

Any such resolution would be symbolic, meaning the U.S. and others seeking curbs on Iran's nuclear activities would have to turn to other options.

And those are limited.

President Bush's administration is unlikely to opt for a military strike as it counts down to its final days in office. That leaves Goldschmidt's options of tougher sanctions and improved incentives — including a direct U.S. overture to the Tehran leadership — as the most realistic possibility.

Ultimately, any amount of leverage will be useless unless the Tehran leadership decides it is in its own interest to change its nuclear ways, Goldschmidt suggested.

"They have to come to the right solution by themselves," he said.