Iran’s nuclear program, which stumbled badly after a reported cyberattack last year, appears beset by poorly performing equipment, shortages of parts and other woes as global sanctions exert a mounting toll, Western diplomats and nuclear experts say.

The new setbacks are surfacing at a time when Iran faces growing international pressure, including allegations that Iranian officials backed a clumsy attempt to kill a Saudi diplomat in Washington. Analysts say Iran has become increasingly frustrated and erratic as political change sweeps the region and its nuclear program struggles.

Although Iran continues to stockpile enriched uranium in defiance of U.N. resolutions, two new reports portray the country’s nuclear program as riddled with problems as scientists struggle to keep older equipment working.

At Iran’s largest nuclear complex, near the city of Natanz, fast-spinning machines called centrifuges churn out enriched uranium. But the average output is steadily declining as the equipment breaks down, according to an analysis of data collected by U.N. nuclear officials.

Iran has vowed to replace the older machines with models that are faster and more efficient. Yet new centrifuges recently introduced at Natanz contain parts made from an inferior type of metal that is weaker and more prone to failure, according to a report by the Institute for Science and International Security, a Washington nonprofit group widely regarded for its analysis of nuclear programs.

“Without question, they have been set back,” said David Albright, president of the institute and a former inspector for the U.N. nuclear watchdog, the International Atomic Energy Agency. Although the problems are not fatal for Iran’s nuclear ambitions, they have “hurt Iran’s ability to break out quickly” into the ranks of the world’s nuclear powers, Albright said.

U.S. intelligence officials have concluded that Iran’s clerical leaders are seeking to rapidly acquire the technical capability to make nuclear weapons, though there are indications that top officials have not yet firmly committed to building the bomb. Iran maintains that its nuclear intentions are peaceful.

Western diplomats and nuclear experts say Iranian officials have been frustrated and angered by the program’s numerous setbacks, including deadly attacks on Iranian nuclear scientists. Four Iranian scientists have been killed by unidentified assailants since 2007, and a fifth narrowly escaped death in an attempted car-bombing.

Feeling besieged

Some U.S. officials have suggested that the alleged plot to assassinate Saudi Arabia’s ambassador to Washington was emblematic of the frustration and disarray within Iran’s ruling elite at a time when internal unrest has destabilized the nation’s closest Arab ally, Syrian President Bashar al-Assad.

U.S. officials have said that the alleged assassination plot originated from elements within Iran’s elite Quds Force, a covert paramilitary group. But it is not clear whether the nation’s top leaders knew about or approved the plan.

The alleged $1.5 million scheme fell apart when an Iranian American accomplice sought to hire a Mexican hit man who in reality was an undercover informant for the Drug Enforcement Administration.

“It could be an outgrowth of the fact that we’ve crossed a red line in the Iranians’ eyes,” said a senior administration official involved in high-level discussions of Iran policy.

“We’re used to seeing them do bad things, but this plot was so bizarre, it could be a sign of desperation, a reflection of the fact that they’re feeling under siege,” said the official, who spoke on the condition of anonymity so he could discuss the matter candidly.

Albright noted that Iran has behaved erratically in other arenas as well, using novel tactics to try to gain advanced materials and technology for its nuclear program and weapons systems.

“Their procurement efforts are less thought-through, and they’re getting caught a lot more, which suggests that they are becoming more desperate,” he said.

The Obama administration has sought to use the revelations of the alleged plot to rally international support for stronger sanctions and other measures to discourage Iran from seeking to become a nuclear power.

In Tehran, officials said Monday that they were ready to investigate allegations by the United States that the Quds Force plotted to kill Saudi Ambassador Adel al-Jubeir. “We are ready to patiently investigate any issue, even if it’s fabricated,” Foreign Minister Ali Akbar Salehi told the state-run Islamic Republic News Agency. “We also asked America to give us the information related to this scenario.”

Salehi and other Iranian officials, however, continued to maintain that Iran had nothing to do with the alleged plot, which they dismissed as a “bad Hollywood script.” The plot allegations have seriously strained Iran’s already fragile relations with the United States and Saudi Arabia.

In an interview on al-Jazeera English on Monday, Iranian President Mahmoud Ahmadinejad said the Obama administration made the allegations to divert attention from its own economic problems.

“Why has the U.S. administration leveled this accusation?” Ahmadinejad said. “The truth will be revealed in the end.”

Sharp decline in output

The studies of Iran’s struggling uranium program draw on data collected by U.N. officials who conduct regular inspections of Iran’s facilities to ensure that the nation is not diverting the enriched product into a military weapons program.

The inspectors’ report documented a sharp drop in output in 2009 and 2010, providing the first confirmation of a major equipment failure linked to a computer virus dubbed Stuxnet. Western diplomats and nuclear experts say Stuxnet’s designer intended to attack and disable thousands of first-generation centrifuges at Natanz, undercutting Iran’s ability to make a nuclear bomb. Many experts suspect Israel created the virus, perhaps with U.S. help, but neither nation has acknowledged any role.

Iranian scientists replaced more than 1,000 crippled machines. Afterward the Natanz plant appeared to quickly recover, and production rates soared to surpass levels seen before the attack. Yet, the gains have not lasted, according to the analysis by the Institute for Science and International Security.

Although Iran has managed to squeeze enriched uranium from the plant at a consistent rate, it needs many more centrifuges to produce the amount of enriched uranium the plant was making two years ago.

The decline could stem from the lingering effects of the cyberattack, or it could indicate that Iran’s centrifuges are simply wearing out. In any case, the decline is so significant that Natanz is incapable of fulfilling the needs of the country’s only nuclear power plant, the report said.

Iran has boasted about the performance of its next-generation centrifuges, which its scientists began installing over the summer. The upgraded equipment — at least four times as efficient as the older models — were to be installed at Natanz and in a bunker near the ancient city of Qom, where they would be less vulnerable to airstrikes.

In prototypes, critical components of the machines were made of a high-strength metal known as maraging steel. But the machines that arrived at Natanz in recent weeks had parts made of a less robust material known as carbon fiber, according to the Institute for Science and International Security.

Correspondent Thomas Erdbrink in Tehran contributed to this report.