About 140 cases of missing or unauthorised use of nuclear and radioactive material were reported to the U.N. atomic agency in 2013, highlighting the challenges facing world leaders at a nuclear security summit next week.
Any loss or theft of highly enriched uranium, plutonium or different types of radioactive sources is potentially serious as al Qaeda-style militants could try to use them to make a crude nuclear device or a so-called "dirty bomb", experts say.
Denis Flory, deputy director general of the International Atomic Energy Agency (IAEA), said most of the reported incidents concerned small quantities of radioactive material.
But, "even if they can't be used for making a nuclear weapon, they can be used in radioactive dispersal devices, which is a concern," Flory told Reuters in an interview.
In a "dirty bomb", conventional explosives are used to disperse radiation from a radioactive source, which can be found in hospitals, factories or other places that may not be very well protected.
Holding a third nuclear security summit since 2010, leaders from 53 countries - including U.S. President Barack Obama - are expected to call for more international action to help prevent radical groups from obtaining atomic bombs.
At the March 24-25 meeting in The Hague, they will say that much headway has been made in reducing the risk of nuclear terrorism but also make clear that more must be done to ensure that dangerous substances don't fall into the wrong hands.
The Dutch hosts say the aim is a summit communique "containing clear agreements" to prevent nuclear terrorism by reducing stockpiles of hazardous nuclear material, better securing such stocks and intensifying international cooperation.
Flory said member states had reported a total of nearly 2,500 cases to the IAEA's Incident and Trafficking Database since it was set up two decades ago. More than 120 countries take part in this information exchange project, covering theft, sabotage, unauthorised access and illegal transfers.
NUCLEAR SECURITY PACT DELAYED
In 2012, 160 incidents were reported to the IAEA, of which 17 involved possession and related criminal activities, 24 theft or loss and 119 other unauthorised activities, its website says.
"It is continuing, which means there is still a lot of work to do to have that really decrease," Flory said with respect to the statistics. However, there are also "more and more countries which declare incidents. The number of incidents we don't know is probably decreasing."
Because radioactive material is less hard to find and the device easier to make, experts say a "dirty bomb" - which could cause panic and have serious economic and environmental consequences - is a more likely threat than a deadly atom bomb.
Radical groups could theoretically build a crude nuclear bomb if they had the money, technical knowledge and fissile materials needed, analysts say.
One of the biggest challenges ahead is to finally bring into force a 2005 amendment to the Convention on Physical Protection of Nuclear Materials (CPPNM), Flory said.
There are still 27 countries - including the United States - which need to ratify the amendment, which expands the coverage from only the protection of nuclear material in international transport to also include domestic use, transport and storage.
"It is extremely important because this amendment brings a lot of strengthening in the field of nuclear security," he said.
Harvard University professor Matthew Bunn said this month that a U.S. failure so far to ratify the amended convention "has made it far harder" for Washington to pressure others to do so.
"The problem appears to be a combination of lack of sustained high-level attention by both the administration and Congress and disputes over unrelated issues," Bunn said.
Flory, who heads the IAEA's nuclear safety and security department, said he knew that the U.S. administration was "very keen on finishing the process" as soon as possible.
"This is a country where you have a lot of nuclear material, a lot of nuclear facilities and they have a lot of influence on nuclear security."