Iran dramatically increased its production of a more enriched form of uranium in recent months, U.N. inspectors reported Thursday, suggesting that the country’s nuclear facilities were ramping up their output even as Iran’s leaders engaged in international negotiations on possible curbs to its nuclear program.

At the same time, Iran appeared to have taken steps that would make it harder to use its uranium stockpile to make nuclear bombs, the International Atomic Energy Agency reported.

The mixed picture of Iran’s nuclear progress prompted renewed calls from the Obama administration for a diplomatic resolution to the standoff that has led to oil embargoes, economic sanctions and threats of military strikes.

“The window of opportunity to resolve this diplomatically remains open, but it will not remain open indefinitely,” White House spokesman Jay Carney told reporters after the IAEA report was released.

The report, based on routine monitoring of Iran’s nuclear facilities, documented a sizable jump in Iran’s stockpile of uranium enriched to 20 percent, a level that can be converted relatively easily to the more highly enriched uranium needed for weapons. The report said Iran has 255 pounds of uranium enriched at 20 percent, up from 159 pounds in May.

But the IAEA also found that Iran had converted much of the new material to metal form for use in a nuclear research reactor. Once the conversion has taken place, the uranium can’t be further enriched to weapons-grade material, Obama administration officials said.

“Production is steadily growing, but it is not as if this material is available for a nuclear weapons ‘break-out,’  ” said a senior official who insisted on anonymity in discussing the administration’s private assessments of the IAEA report.

The IAEA also confirmed that Iran has added more than 1,000 centrifuges to an underground enrichment plant near the city of Qom. However, inspectors discovered that the majority of the new machines were not yet working

Iran insists that the uranium is intended to fuel the country’s sole medical research reactor, although its existing stockpile far exceeds the reactor’s needs for years to come.

The IAEA report chided Iranian officials for continuing to enrich uranium in defiance of U.N. resolutions and said Iran has not provided convincing assurances about the peaceful intentions of its nuclear program. “Iran is not providing the necessary cooperation,” said the report, prepared for the U.N. agency’s 35-nation governing board.

U.N. officials also criticized Iran for refusing to grant IAEA inspectors access to facilities where Iranian scientists are
suspected to have carried out nuclear-weapons related experiments. Tehran has sparred for months with the U.N. nuclear watchdog over whether inspectors can examine a military site known as Parchin, where Iran is believed to have tested a type of detonator used to trigger nuclear explosions.

Months of negotiations failed to produce an agreement on access to Parchin, and U.N. officials cited satellite photographs showing that cleanup work has been underway there.

“Significant ground-scraping and landscaping have been undertaken over an extensive area at and around the location,” and several buildings have been demolished or covered with large shrouds, the report said.

The release of the IAEA’s latest report card on Iran came as the country’s supreme leader repeated his assertion that nuclear weapons are contrary to Islamic teaching.

Ayatollah Ali Khamenei told a gathering of the 120-nation Non-Aligned Movement that acquiring atomic bombs was a “big and unforgivable sin.” But he also insisted that Iran would never succumb to Western pressure.

“I declare that the Islamic Republic of Iran has never been after nuclear weapons and it will never abandon its right for peaceful use of nuclear energy,” Khamenei told delegates in Tehran.