The head of the UN nuclear watchdog agency warned Monday of the threat of violent extremists getting their hands on radioactive material to make "dirty" bombs.

Yukiya Amano, director general of the International Atomic Energy Agency (IAEA), said all countries should secure material which could be used to make such a device.

"Nuclear security is a longstanding and real issue. This is a real threat in all countries where nuclear material is available to terrorists," he said during a visit to the Philippines for an atomic energy conference.

"If nuclear material falls into the hands of terrorists, that can be used for dirty bombs," he said, referring to devices that use explosives to spread radioactive material over a wide area to poison people.

Amano said such devices, which are much easier to make than an atomic bomb, could cause widespread panic if unleashed in a city.

While governments have the main responsibility to secure these materials, the IAEA can help establish guidelines to guard them, he told reporters.

This could include training customs and border guards on how to detect such materials and even helping governments procure detectors and similar equipment.

Amano said the Vienna-based IAEA was already maintaining a database to "analyse and establish a trend" that might identify possible threats.

Despite the danger, Amano said nuclear energy had many benefits beyond even power generation, such as use in medicine, water management and even agriculture.

Concern about possible terror attacks has risen sharply worldwide recently following a spate of bombings and mass shootings by supporters of the Islamic State group.