"Radiation protection experts of the Austrian Research Centers (ARC) confirmed the initial findings from the laboratory's automatic monitoring system which indicated that there had been no release of radioactivity to the environment," the International Atomic Energy Agency said in a statement.

"Since the incident, constant air monitoring near the laboratory, undertaken by the IAEA, has also provided no evidence of any radioactive contamination."

On August 3, there was a plutonium leak in the IAEA's ageing laboratory in Seibersdorf, 35 kilometres (more than 20 miles) southeast of Vienna.

The incident followed the IAEA's own warnings late last year that the facility was outdated and did not meet safety standards.

In its statement on Friday, the IAEA said a tiny amount of plutonium contained in an acid solution had "spilled from five small glass vials when one of them burst after a build up of pressure in it. The vials were stored in a secure steel safe."

In all, there was less than one gramme of plutonium in the five vials, the watchdog said.

The material was in the laboratory for scientific reference purposes and virtually all of the contamination was confined within the steel-walled safe.

"As previously reported, the automatic alarm was triggered when highly sensitive detectors of the continuous air monitoring system identified minor amounts of radioactive aerosols in the storage room containing the safe," the statement said.

"The air contamination was trapped entirely in the filters of the ventilation system. No one was working in the laboratory at the time of the accident, which occurred at 2:31 am (0031 GMT)."

An investigation into the circumstances and causes of the incident was still underway, the IAEA continued.

"In the meantime, the first stage of the clean-up of the storage room was successfully completed on Friday, 22 August."

According to an assessment of the incident, the IAEA's nuclear regulator had found that the lab's safety systems "worked properly and successfully contained the contamination."

The incident was rated as level 1 (anomaly) on the Nuclear and Radiological Event Scale (INES) scale of events. The INES scale has seven categories, the most serious being a "major accident."

The IAEA's laboratories in Seibersdorf provide research and training in applying nuclear science to environmental protection, insect pest control, plant breeding, human and animal health, as well as physical and chemical studies, and nuclear instrumentation.

The Safeguards Analytical Laboratory, where the incident occurred, undertakes most of the IAEA's analysis of nuclear material samples (very small quantities of uranium or plutonium), collected by its safeguards inspectors from civilian nuclear facilities, as part of its normal verification work.