A gas leak sparked an explosion and fire Tuesday at a nuclear plant that is believed to produce enriched plutonium for Pakistan's atomic weapons program. Two workers were killed.

The Khushab heavy water plant was shut down while undergoing annual maintenance at the time, the Pakistan Atomic Energy Commission said. The commission evacuated the plant but assured the public that there was no risk of exposure outside the sprawling facility southeast of the capital, Islamabad.

The "situation was immediately brought under control, and two workers lost their lives while controlling the incident," the commission said.

It said the leaking gas was burned off by plant equipment and that the cause was under investigation.

The government claims Khushab produces electricity. Last year, the Washington-based Institute of Science for International Security said the plant has three reactors, including two that were still under construction last June. It cited satellite photos of the sprawling site that is under military control.

The development of the reactor and other nuclear-related activities "imply" that Pakistan has decided to "increase significantly its production of plutonium for nuclear weapons," the institute said in a report analyzing the images.

Police near the plant said they were advised by plant officials to prepare buses for a wider evacuation, but then were told they would not be needed.

Hamid Mukhtar Gondal, police chief for the district where Khushab is located, said he was told that an accidental blast was caused by cylinders of an unspecified gas.

"After the blast, the building caught fire," Gondal told The Associated Press. "Two men sustained burns and died on the way to a hospital.

"At the moment, with the help of God, everything is under control," he said. "The fire has been put out. There is no spread of poisonous gas at all."

Ghulam Muhammad, the mayor of the neighboring town of Khushab, said there was initial panic as the plant and the residential colony for workers were evacuated and roadblocks thrown up to cordon off the area.

Within three hours, plant management gave an all clear and removed the roadblocks, he said, adding he was unhappy that local officials were not immediately told of the incident.

"We only got informed of the gas leak by the employees rushing out of the colony," he said. "For public safety, they need to inform the local administration."

Pakistan, which carried out nuclear tests in 1998 to match those by archrival India, has a fairly safe nuclear safety record. But some minor incidents have occurred in recent years.

In 2003, one laboratory worker was killed and another man injured in an explosion in a liquid nitrogen tank at the Pakistan Institute of Engineering and Applied Sciences near Islamabad.