A gas leak in Pakistan has killed two people at a heavy water plant run by the country's atomic energy agency in Punjab province, officials say.
They say that the leak at the Khushab heavy water plant happened when it was closed for annual maintenance.
It is believed to be the first fatal accident at any of the country's nuclear facility.
The main function of the plant at Khushab is the production and refinement of plutonium.
A spokesman said that an inquiry had been ordered.
Pakistan built its first nuclear power station in 1972 in Karachi with the help of Canadian experts.
But Western countries, lobbied by the US, later halted cooperation amid fears that Pakistan was secretly developing nuclear weapons.
"The Khushab heavy water plant was under annual maintenance and was under shutdown status," the Pakistan Atomic Energy Commission said in a statement.
"All necessary steps were taken, including evacuation of personnel as a precautionary measure.
"The situation was immediately brought under control, and two workers lost their lives while controlling the incident. There is no threat to public life."
It said the leaking gas has now been burned off.
The plant at Khushab is not subject to International Atomic Energy Agency safeguards, unlike other nuclear plants in Karachi and at Chashma in Punjab.
The BBC's Syed Shoaib Hasan is Islamabad says that it is Pakistan's most well-protected fissile material production facility.
Our correspondent says that the country's premier plant was the source of much tension between the US, Pakistan and China when it was made operational.
The US was highly critical of the project which, it said, would exacerbate the nuclear weapons race in South Asia.
Pakistan conducted five nuclear weapon tests in May 1998 to become a recognised nuclear-armed state.