WASHINGTON - Iran has increased its stockpile of 20 percent enriched uranium by nearly a third since May, U.N. investigators reported Thursday, signaling that Tehran is pushing ahead with nuclear development despite tightening U.S. and European sanctions and the threat of an Israeli military strike.

The International Atomic Energy Agency, the U.N.'s nuclear watchdog agency, also reported that Iran has doubled the number of centrifuges, which are used to enrich uranium, in an underground bunker near the holy city of Qom that experts say has been built to withstand an attack.

The findings add to U.S. and Israeli concerns that the Islamic Republic is fast expanding its enrichment efforts even as it negotiates with world powers seeking to curb the program before Iran can develop the capability to build a nuclear weapon. Iran insists that its nuclear program is for peaceful purposes.

The negotiations, which began last spring, have stalled but have not shut down.

"The fact that they have accelerated their activities seems to suggest that they're using this period of a diplomatic lull to bring as many facilities and centrifuges online as possible," said Ray Takeyh, an Iran expert at the nonpartisan Council on Foreign Relations. "That way, when they do (re-enter) negotiations, whenever that is, they can say, 'What's here is here. Let's talk about the future.'"

Israeli leaders have warned that Iran is approaching a so-called zone of immunity in which an airstrike wouldn't sufficiently damage Tehran's ability to build a nuclear weapon. Israeli Prime Minister Benjamin Netanyahu said Thursday that he will deliver a speech on Iran's "terror regime" at the U.N. General Assembly next month.

The White House said it is determined not to let Iran develop a nuclear bomb, but it also remains committed to a policy of steadily tightening sanctions that have blocked Iran from importing nuclear materials and squeezed its oil exports _ its main source of income _ by half.

"The window of opportunity to resolve this diplomatically remains open, but it will not remain open indefinitely," said White House press secretary Jay Carney. "So long as the Iranian regime refuses to comply with its international obligations, the United States, with its allies, will continue to take actions to further isolate and penalize Iran and the regime."

The IAEA report, the outlines of which were first reported last week, found that Iran by mid-August had produced a total of nearly 418 pounds of uranium enriched to 20 percent purity, a level that would allow it to rapidly produce the 80 percent enriched fuel needed for a weapon. That figure marked an increase of 31 percent from mid-May.

U.N. investigators reported that nearly three-quarters of the roughly 2,800 planned centrifuges, the fast-spinning machines that refine uranium, have been installed at Fordow, enrichment facility being built secretly built under a mountain and discovered by Western intelligence in 2009. A total of 2,140 centrifuges have been installed at Fordow - up from about 1,000 - although not all were operating.

Investigators also accused Tehran of continuing to dismantle parts of a large military-industrial facility that some analysts believe had been used to conduct nuclear experiments. Iran has consistently denied inspectors access to Parchin, south of the capital, and told the nuclear agency in a letter this week that allegations of nuclear activity there were "baseless."

Satellite images captured since February showed that five structures at the site had been demolished, power lines and paved roads had been removed and "significant ground scraping and landscaping" had occurred, the report says. Investigators said that these "extensive activities" would make it difficult for them to determine whether Iran is telling the truth about the facility.

"The IAEA is facing real trouble with Parchin," said David Albright, a nonproliferation expert and president of the nonprofit Institute for Science and International Security. "Iran has really refused to let them in and appears intent on a complete cleanup before they do. And then it's too late for the IAEA to find out anything about what went on there."

Iran also continued to expand its stockpile of low-enriched uranium at its main enrichment facility, Natanz, which apparently has recovered from a computer virus attack. The virus, Stuxnet, is believed to have been developed by the U.S. and Israel to sabotage Iranian centrifuges.

U.S. intelligence officials believe that Iran hasn't made the decision to build a nuclear bomb, and it's unclear whether Tehran has figured out the other components needed for a weapon, such as trigger mechanisms.

Iran's supreme leader, Ayatollah Ali Khamenei, addressing an international summit of nonaligned nations Thursday in Tehran, said that "we proposed the idea of a Middle East free of nuclear weapons, and we are committed to it. This does not mean forgoing our right to peaceful use of nuclear power and production of nuclear fuel."

Experts say that developing the fissile material is the most difficult part of a nuclear program and that, as Iran brings new centrifuges online, it is shortening the time it would need to produce a weapon.

"They're doing it better and better, and nothing seems to suggest that either the economic sanctions or the sanctions aimed at preventing them from getting nuclear materials have been thwarting them," said Blaise Misztal, an Iran expert at the Bipartisan Policy Center in Washington. "They're definitely not slowing down and not showing much signs of slowing down."

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