The head of the UN atomic agency warned 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 in Vienna of some 1,200 delegates from around 110 states including 35 ministers 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.

"Nuclear terrorism" comprises three main risks: an atomic bomb, a "dirty bomb" -- conventional explosion spreading radioactive material -- and an attack on a nuclear plant.

The first, using weapons-grade uranium or plutonium, is generally seen as "low probability, high consequence" -- very difficult to pull off but for a determined group of extremists, not impossible.

There are hundreds of tonnes of weapons-usable plutonium and uranium -- a grapefruit-sized amount is enough for a crude nuclear weapon that would fit in a van -- around the world.

A "dirty bomb" -- a "radiological dispersal device" or RDD -- is much easier but would be hugely less lethal. But it might still cause mass panic.

"If the Boston marathon bombing (in April this year) had been an RDD, the trauma would be lasting a whole lot longer," Sharon Squassoni from the Center for Strategic and International Studies (CSIS) told AFP.

Last year alone, the IAEA recorded 17 cases of illegal possession and attempts to sell nuclear materials and 24 incidents of theft or loss. And it says this is the "tip of the iceberg".

Many cases have involved former parts of the Soviet Union, for example Chechnya, Georgia and Moldova -- where in 2011 several people were arrested trying to sell weapons-grade uranium -- but not only.

Nuclear materials that could be used in a "dirty bomb" are also used in hospitals, factories and university campuses and are therefore seen as easy to steal.

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 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 in March.

A report issued in Vienna on Monday to coincide with the start of the meeting by the Arms Control Association and the Partnership for Global Security said decent progress had been made but that "significant" work remained.

Ten countries have eliminated their entire stockpiles of weapons-grade uranium, many reactors producing nuclear medicines were using less risky materials and smuggling nuclear materials across borders, for example from Pakistan, is harder, it said.

But some countries still do not have armed guards at nuclear power plants, security surrounding nuclear materials in civilian settings is often inadequate and there is a woeful lack of international cooperation and binding global rules.

"We are still a long way from having a unified regime, a unified understanding of the threat and a way to address it," Michelle Cann, co-author of the report, told AFP.