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Iran told atomic inspectors this week that it had run into a serious problem at a newly completed nuclear reactor that was supposed to start feeding electricity into the national grid this month, raising questions about whether the trouble was sabotage, a startup problem, or possibly the beginning of the project’s end.

In a report on Friday, the International Atomic Energy Agency said Iran told inspectors on Wednesday that it was planning to unload nuclear fuel from its Bushehr reactor — the sign of a major upset. For years, Tehran has hailed the reactor as a showcase of its peaceful nuclear intentions and its imminent startup as a sign of quickening progress.

But nuclear experts said the giant reactor, Iran’s first nuclear power plant, now threatens to become a major embarrassment, as engineers remove 163 fuel rods from its core.

Iran gave no reason for the unexpected fuel unloading, but it has previously admitted that the Stuxnet computer worm infected the Bushehr reactor. On Friday, computer experts debated whether Stuxnet was responsible for the surprising development.

Russia, which provided the fuel to Iran, said earlier this month that the worm’s infection of the reactor should be investigated, arguing that it might trigger a nuclear disaster. Other experts said those fears were overblown, but noted that the full workings of the Stuxnet worm remained unclear.

In interviews Friday, nuclear experts said the trouble behind the fuel unloading could range from minor safety issues and operational ineptitude to serious problems that would bring the reactor’s brief operational life to a premature end.

“It could be simple and embarrassing all the way to ‘game over,’ ” said David A. Lochbaum, a nuclear engineer at the Union of Concerned Scientists and a former official at the Nuclear Regulatory Commission, which oversees nuclear reactors in the United States.

Mr. Lochbaum added that having to unload a newly fueled reactor was “not unprecedented, but not an everyday occurrence.” He said it happened perhaps once in every 25 or 30 fuelings. In Canada, he added, a reactor was recently fueled and scrapped after the belated discovery of serious technical problems.

“This could represent a substantial setback to their program,” David Albright, president of the Institute for Science and International Security, a private group in Washington that tracks nuclear proliferation, said of the problem behind the Bushehr upset.

“It raises questions of whether Iran can operate a modern nuclear reactor safely,” he added. “The stakes are very high. You can have a Chernobyl-style accident with this kind of reactor, and there’s lots of questions about that possibility in the region.”

The new report from the I.A.E.A. — a regular quarterly review of the Iran nuclear program to the agency’s board — gave the reactor unloading only brief mention and devoted its bulk to an unusually toughly worded indictment of Iranian refusals to answer questions about what the inspectors called “possible military dimensions” of its nuclear program.

The report alluded to “new information recently received,” suggesting continuing work toward a nuclear warhead.

But the inspectors provided no details about the new information or how it was received. The I.A.E.A. frequently gets its data from the intelligence agencies of member countries, including the United States, but it also tries to collect data from its own sources.

The report on Friday referred directly to concerns that Iran was working on “the development of a nuclear payload for a missile.” But it noted that all of its requests for information had been ignored for years, with Iranian officials arguing that whatever information the agency possessed, it was based on forgeries.

The White House said Friday that the report cast new light on what it called Iran’s covert movement toward nuclear arms.

“The I.A.E.A.’s reports of obstruction and Iran’s failure to cooperate are troubling,” said Tommy Vietor, spokesman for the National Security Council. “We will continue to hold Iran accountable to its international nuclear obligations, including by deepening the international pressure on Iran.”

The reactor is located outside the Iranian city of Bushehr on the nation’s Persian Gulf coast. Priced at more than a billion dollars, it is ringed by dozens of antiaircraft guns and large radar stations meant to track approaching jets.

Its tangled history began around 1975 with a West German contract. After the Islamic Revolution in 1979, the West Germans withdrew. Iraq repeatedly bombed the half-built reactor between 1984 and 1988.

Iran signed a rebuilding accord with Russia in 1995 that should have had the project completed in 1999. But the plan bogged down in long delays.

The United States once opposed the plant. But Washington dropped its objections after Russia agreed to take back the spent rods, removing the possibility that Iran could reprocess them for materials that could fuel nuclear arms.

The loading of uranium fuel into the reactor was initially planned to start soon after its shipment to Bushehr last August, but was delayed by what the Iranians said was a leak in a pool near the central reactor.

In October, Iranian officials said the Stuxnet worm had infected the reactor complex, but they played down the issue. Mohammad Ahmadian, an Iranian Atomic Energy Organization official, said the affected computers had been “inspected and cleaned up.”

Later in October, as the fueling at last got under way, after three decades of delay, the head of Iran’s Atomic Energy Organization, Ali Akbar Salehi, called the Bushehr reactor “the most exceptional power plant in the world.”

In December, he predicted that the plant would be connected to the national power grid by Feb. 19. “This phase,” he said, according to The Tehran Times, “is the most important operational work of the plant.”

In an interview on Friday, a European diplomat familiar with Iran’s nuclear program called the fueling problem a major setback, even if the technical cause proves to be less than monumental.

“It’s clearly a significant setback to the startup of the reactor,” said the diplomat, who spoke on the condition of anonymity because of the diplomatic delicacy of the matter.

He said that engineers at Bushehr had identified a technical failure, but were struggling to understand its cause.

“It’s too early to know,” the diplomat said. “I’m sure the Iranians are studying that question quite desperately.”