A gas leak killed two people on Tuesday at a heavy water plant run by Pakistan's atomic energy agency in central Punjab province, a spokesman for the agency said.
The leak at the Khushab heavy water plant took place during its annual maintenance. The plant was closed at the time.
"The situation was immediately brought under control and two workers lost their lives while controlling the incident," a spokesman for Pakistan Atomic Energy Commission (PAEC) said.
He said all necessary steps had been taken to protect the plant, including evacuation of the personnel as a precautionary measure.
The spokesman said an inquiry had been ordered.
Asked if the leak was unconnected to any nuclear facility, the PAEC spokesman said there was no radioactive leak.
"No radiation was involved. It was some chemical gas. The situation is fully under control," he said.
"There is no threat to the public as all the leaking gas has been burnt in the flare system of the plant."
Pakistan built its first nuclear power station in 1972 in Karachi with Canadian help. But Western countries, under pressure from the United States, later halted cooperation amid suspicions that Pakistan was secretly developing nuclear weapons.
The atomic energy agency also runs a Chinese-supplied nuclear plant in Chashma in Punjab. A third plant is being constructed at Chashma.
The nuclear facilities at Karachi and Chashma are subject to IAEA safeguards, unlike those at Khushab.
Pakistan conducted five nuclear weapons' tests in May 1998 in response to those of rival India, to become a nuclear-armed state.
The Commission runs a nuclear facility at Khushab.
The Washington-based Institute for Science and International Security reported last year that Pakistan was building a third plutonium production reactor at Khushab, though the government has not confirmed this.