By Tim Hirsch BBC Environment Correspondent

Fears about the possible risks of radioactive material being used by terrorists have tended to focus on the poor security in place in those countries which used to make up the Soviet Union and its satellite states.

But Tuesday's report by the International Atomic Energy Agency (IAEA) says the problem goes much wider.

It says more than 100 countries lack the controls to prevent or even detect the theft of these materials.

The danger of handling powerful radioactive sources can no longer be seen as an effective deterrent

Mohamed El Baradei, IAEA

Millions of sources of radioactivity have been used widely for decades for functions unconnected with the nuclear industry, from diagnosing illnesses to monitoring oil wells.

While most of these do not give off enough radiation to cause serious harm, the fear is that with thousands outside the control of national or international authorities, it would not be difficult for terrorist groups to obtain the materials needed to set off a so-called "dirty bomb".

Even if the substances used did not in fact present a genuine threat to health, the detonation of any device containing radioactive material may well create the social and psychological disruption which is the main aim of terrorists.

Witness the huge political and economic impact of last year's attack on New York which went way beyond the actual physical damage and loss of life.

No complacency

The IAEA report guards against complacency in advanced democracies that this is a problem "out there" in countries with weak systems of regulation.

Even in the United States, it says, nuclear regulators report that since 1996, companies have lost track of 1,500 sources of radioactivity, and less than half have since been recovered.

The European Commission recently estimated up to 70 such sources were lost each year, with a further 30,000 disused materials in danger of escaping the clutches of the authorities.

A black market trade in radioactive substances could increase

But it is in the former communist bloc and the developing world that actual cases of radioactive contamination, most of them accidental, have been documented.

The worst of these took place in the Brazilian city of Goiania in 1987.

Scavengers dismantled a metal canister from a radiotherapy machine at an abandoned cancer clinic, and left it in a junkyard.

This released the radioactive substance caesium 137 which contaminated more than 200 people in the city.

Four subsequently died.

Some children and adults even rubbed the deadly powder on their bodies, thinking it looked pretty.

The only known deliberate attempt to use this kind of material to harm people was in 1996, when Chechen rebels left a container of caesium 137 in a Moscow park, fortunately it did not create any contamination.

Stricter controls

But the IAEA warns of a widespread black market trade in radioactive substances.

Most of it is carried out for the value of the metals themselves but an "important fraction" involves people who expect to find buyers interested in the terror value of the material.

The agency's director general Mohamed El Baradei warns that if people are prepared to ignore their own personal safety, it becomes much easier to smuggle these substances concealed in a lorry or packed in a suitcase.

"The danger of handling powerful radioactive sources can no longer be seen as an effective deterrent, which dramatically changes previous assumptions," said Mr El Baradei.

In the US less than half of lost radioactivity sources have been recovered

The IAEA said there had been improvements in international co-operation to strengthen control over radioactive material, but much more needed to be done.

It is unrealistic to imagine that controls will be tight enough in the foreseeable future to prevent a determined terrorist from laying his hands on the material needed for a "dirty bomb".

Years of neglect of this problem have provided a chilling dimension to the potential arsenal of groups looking for new ways to create havoc.