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."