Yukiya Amano is director general of the International Atomic Energy Agency.

The International Atomic Energy Agency is aware of more than 2,000 confirmed cases of illicit trafficking and other unauthorized activities involving nuclear and other radioactive material in the past 18 years. In a sting operation in Moldova last year, police seized a quantity of highly enriched uranium — material that can be used in a nuclear weapon — from an individual who was trying to sell it.

Most cases of attempted trafficking do not involve nuclear materials but, rather, radioactive materials of the sort held in hospitals, factories and many other locations worldwide that are generally not as well protected as nuclear facilities.

To help avoid the deaths, injuries, mass panic, widespread contamination and major economic and social disruption that would result if terrorists sabotage a nuclear reactor or detonate a “dirty bomb,” it is vital that world leaders attending the Nuclear Security Summit in Seoul this week agree to strengthen measures to prevent nuclear and other radioactive material from falling into the wrong hands.

Progress has been made since President Obama hosted the first such summit two years ago. But nuclear and other radioactive material is still inadequately secured in some countries. There is a real risk of terrorists acquiring and using such material. This global threat requires a global response. Criminals do not respect national borders. Neither does ionizing radiation.

It is urgent that all countries ratify the Amendment to the Convention on the Physical Protection of Nuclear Materials, a crucial anti-terrorism instrument. Effective physical protection of materials is the first line of defense against malicious acts.

The amendment makes it legally binding for countries to protect nuclear material when it is being used or stored, not just when it is being transported — as the convention currently stipulates — and would require them to protect nuclear facilities against acts of sabotage that could have consequences similar to nuclear accidents.

Agreement was reached on the amendment in 2005, but it has not entered into force because not enough countries have ratified it. More than 20 countries attending the Seoul summit have not taken this indispensable step. That needs to change.

It was once thought that nuclear and radiological materials were self-protecting because they are dangerous to handle without specialist training and equipment. But individuals and groups engage in illicit trafficking despite risk to their own and others’ health. The fact that the smugglers in Moldova had tried to evade detection by building a shielded container represents a worrying level of sophistication. Fortunately, Moldovan authorities had the capabilities to detect the materials; the uranium was seized and arrests were made.

In one of the world’s worst radiological incidents, radioactive material stolen from a disused clinic in Goiania, Brazil, in 1987 caused the deaths of four people, while nearly 300 suffered radioactive contamination and more than 100,000 sought radiological screening.

That incident involved the unintended release of radioactivity, but it remains the best real-world indicator of what could happen on a larger scale if terrorists were to detonate a dirty bomb in a large city or at a major public event.

I do not wish to be alarmist. Progress continues to be made in protecting vulnerable material and establishing effective border controls. But more needs to be done. Failure to bring the Amendment to the Convention on the Physical Protection of Nuclear Materials into force is a serious shortcoming in the global security framework. I urge world leaders to put this right when they meet in Seoul.