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TEHRAN, Iran - In a major setback to Iran's nuclear program, technicians will have to unload fuel from the country's first atomic power plant because of an unspecified safety concern, a senior government official said.

The vague explanation raised questions about whether the mysterious computer worm known as Stuxnet might have caused more damage at the Bushehr plant than previously acknowledged. Other explanations are possible for unloading the fuel rods from the reactor core of the newly completed plant, including routine technical difficulties.

While the exact reason behind the fuel's removal is unclear, the admission is seen as a major embarrassment for Tehran because it has touted Bushehr -- Iran's first atomic power plant -- as its showcase nuclear facility and sees it as a source of national pride. When the Islamic Republic began loading the fuel just four months ago, Iranian officials celebrated the achievement.

Iran's envoy to the UN nuclear monitoring agency in Vienna said that Russia, which provided the fuel and helped construct the Bushehr plant, had demanded the fuel be taken out.

"Upon a demand from Russia, which is responsible for completing the Bushehr nuclear power plant, fuel assemblies from the core of the reactor will be unloaded for a period of time to carry out tests and take technical measurements," the semiofficial ISNA news agency quoted Ali Asghar Soltanieh as saying.

"Iran always gives priority to the safety of the plant based on highest global standards," Soltanieh said.

Calls to the Russian nuclear agency Rosatom for comment were not answered Saturday afternoon.

The Bushehr plant is not among the aspects of Iran's nuclear program that are of top concern to the international community and is not directly subject to sanctions. It has international approval and is supervised by the UN's nuclear monitoring agency, the International Atomic Energy Agency.

The IAEA said in a report released Friday about Iran's nuclear program that Tehran informed the agency on Wednesday that it would have to unload the fuel rods. The agency said it and Tehran have agreed on the "necessary safeguards measures."

A senior international official familiar with Iran's nuclear program said the IAEA had no further details. He said unloading and reloading fuel assemblies is not unusual before any reactor startup. The official asked for anonymity because his information was confidential.

Soltanieh and other officials have not specified why the fuel had to be unloaded, but Iranian officials denied any link to the Stuxnet computer virus.

"Stuxnet has had no effect on the control systems at the Bushehr nuclear power plant," Nasser Rastkhah, a senior official in charge of nuclear security, told the official IRNA news agency.

Foreign intelligence reports have said the control systems at Bushehr were penetrated by the malware -- malicious software designed to infiltrate computer systems -- but Iran has all along maintained that Stuxnet was only found on several laptops belonging to plant employees and didn't affect the facility's control systems.

Some computer experts believe Stuxnet was the work of Israel or the United States, two nations convinced that Iran wants to turn nuclear fuel into weapons-grade uranium.

The Islamic Republic is reluctant to acknowledge setbacks to its nuclear activities, which it says are aimed at generating energy but are under UN sanctions because of concerns they could be channeled toward making weapons. Only after outside revelations that its enrichment program was temporarily disrupted late last year by Stuxnet did Iranian officials acknowledge the incident.

The startup of the Bushehr power plant, a project completed with Russian help but beset by years of delays, would deliver Iran the central stated goal of its atomic work — the generation of nuclear power.

But the inauguration of the facility has been delayed for years. Iran said when it began inserting the fuel rods in October that the 1,000-megawatt light-water reactor would begin pumping electricity to Iranian cities by December. But it pushed back the timing to February, citing a "small leak" and other unspecified reasons.

The Bushehr plant itself is not among the West's main worries because safeguards are in place to ensure that the spent fuel will be returned to Russia and cannot be diverted to weapons making.

The United States and some of its allies believe the Bushehr plant is part of a civil energy program that Iran is using as cover for a covert program to develop a nuclear weapons capability. Iran denies the accusation.

The Bushehr project dates back to 1974, when Iran's U.S.-backed Shah Mohammed Reza Pahlavi contracted with the German company Siemens to build the reactor. The company withdrew from the project after the 1979 Islamic Revolution toppled the shah and brought hard-line clerics to power.

In 1992, Iran signed a US$1 billion deal with Russia to complete the project and work began in 1995.

Under the contract, Bushehr was originally scheduled to come on stream in July 1999 but the startup has been delayed repeatedly by construction and supply glitches.