The IAEA said "pressure build-up in a small sealed sample bottle in a storage safe" released a small amount of plutonium at 2:30 a.m. (0030 GMT) on Sunday when no one was in the area.

The leak spread to two neighbouring storage rooms. "All indications are that there was no release of radioactivity to the environment. Further monitoring around the laboratory will be undertaken," an IAEA statement said.

The area was sealed pending decontamination.

An IAEA spokeswoman, however, denied the incident had anything to do with the condition of the laboratory. "This pressure build-up of toxic gases that plutonium produces ... could have happened in the most modern lab as well," she said.

Austria's environment ministry confirmed its monitoring stations had detected no heightened radioactivity.

"The laboratory is equipped with multiple safety systems, including an air-filtering system to prevent the release of radioactivity to the environment," the IAEA statement said.

An investigation of the incident was under way.

Located within the complex of the Austrian Research Centers 35 km (20 miles) southeast of Vienna, the lab analyses samples of nuclear material, mainly plutonium or uranium, taken during agency surveillance missions worldwide.

The IAEA annually analyses some 800 environmental samples collected by inspectors in member states, among them 90 percent of all traces of plutonium, a common atomic bomb fuel.

Last year, IAEA Director Mohamed ElBaradei said the agency's ability to provide timely sample analysis, a cornerstone of efforts to stop diversions of nuclear materials into bomb making, was at risk due to ageing technical infrastructure. (Reporting by Mark Heinrich; editing by Sami Aboudi)