The head of the UN atomic agency warned on Monday against complacency in preventing "nuclear terrorism", saying progress in recent years should not lull the world into a false sense of security.
"Much has been achieved in the past decade," Yukiya Amano of the International Atomic Energy Agency told a gathering of some 1,200 delegates from around 110 states including 35 ministers in Vienna to review progress on the issue.
"Many countries have taken effective measures to prevent theft, sabotage, unauthorised access, illegal transfer, or other malicious acts involving nuclear or other radioactive material. Security has been improved at many facilities containing such material."
Partly as a result, he said, "there has not been a terrorist attack involving nuclear or other radioactive material".
"But this must not lull us into a false sense of security. If a 'dirty bomb' is detonated in a major city, or sabotage occurs at a nuclear facility, the consequences could be devastating.
"The threat of nuclear terrorism is real, and the global nuclear security system needs to be strengthened in order to counter that threat."
"Nuclear terrorism" comprises three main risks: an atomic bomb, a "dirty bomb" - conventional explosion spreading radioactive material - and an attack on a nuclear plant.
Major international efforts have been made since the end of the Soviet Union in 1991 and the September 11, 2001 attacks in the United States a decade later to prevent nuclear material falling into the wrong hands.
US President Barack Obama hosted a summit in 2010 on the subject which was followed by another one in Seoul last year. A third is planned in The Hague next March.